In r118390, the swap pager's approach to striping swap allocation over
multiple devices was changed.  However, swapoff_one() was not fully and
correctly converted.  In particular, with r118390's introduction of a per-
device blist, the maximum swap block size, "dmmax", became irrelevant to
swapoff_one()'s operation.  Moreover, swapoff_one() was performing out-of-
range operations on the per-device blist that were silently ignored by
blist_fill().

This change corrects both of these problems with swapoff_one(), which will
allow us to potentially increase MAX_PAGEOUT_CLUSTER.  Previously,
swapoff_one() would panic inside of blist_fill() if you increased
MAX_PAGEOUT_CLUSTER.

After r118390, the variable "dmmax" was neither the correct strip size
nor the correct maximum block size.  Moreover, after r318995, it serves
no purpose except to provide information to user space through a read-
sysctl.

This change eliminates the variable "dmmax" but retains the sysctl.  It
also corrects the value returned by the sysctl.

Halve the memory being internally allocated by the blist allocator.  In
short, half of the memory that is allocated to implement the radix tree is
wasted because we did not change "u_daddr_t" to be a 64-bit unsigned int
when we changed "daddr_t" to be a 64-bit (signed) int.  (See r96849 and
r96851.)

When the function blist_fill() was added to the kernel in r107913, the swap
pager used a different scheme for striping the allocation of swap space
across multiple devices.  And, although blist_fill() was intended to support
fill operations with large counts, the old striping scheme never performed a
fill larger than the stripe size.  Consequently, the misplacement of a
sanity check in blst_meta_fill() went undetected.  Now, moving forward in
time to r118390, a new scheme for striping was introduced that maintained a
blist allocator per device, but as noted in r318995, swapoff_one() was not
fully and correctly converted to the new scheme.  This change completes what
was started in r318995 by fixing the underlying bug in blst_meta_fill() that
stops swapoff_one() from simply performing a single blist_fill() operation.

Starting in r118390, swaponsomething() began to reserve the blocks at the
beginning of a swap area for a disk label.  However, neither r118390 nor
r118544, which increased the reservation from one to two blocks, correctly
accounted for these blocks when updating the variable "swap_pager_avail".
This change corrects that error.

Originally, this file could be compiled as a user-space application for
testing purposes.  However, over the years, various changes to the kernel
have broken this feature.  This revision applies some fixes to get user-
space compilation working again.  There are no changes in this revision
to code that is used by the kernel.