read-behind / read-ahead support for zfs_getpages()

ZFS caches blocks it reads in its ARC, so in general the optional
pages are not as useful as with filesystems that read the data
directly into the target pages. But still the optional pages
are useful to reduce the number of page faults and associated
VM / VFS / ZFS calls.
Another case that gets optimized (as a side effect) is paging in
from a hole. ZFS DMU does not currently provide a convenient
API to check for a hole. Instead it creates a temporary zero-filled
block and allows accessing it as if it were a normal data block.
Getting multiple pages one by one from a hole results in repeated
creation and destruction of the temporary block (and an associated
ARC header).

Tested with fsx using various supported blocks sizes from 512 bytes
to 128 KB and additionally 1 MB.

Please note that in illumos and ZoL they do not do the range-locking in
the page-in path. This is because ZFS has a double-caching problem
between ARC and page cache and that requires zfs_read() and zfs_write()
to consult pages in the page cache. So, in those functions they first
lock a range and then lock pages corresponding to the range. While in
the page-in (and maybe page-out) path they first lock the pages and then
would lock the range. So, they would have a deadlock.

I believe that FreeBSD does not have that problem, because the page-in
deals only with invalid pages while zfs_read() and zfs_write() need to
access only valid pages. They do not wait on a busy page unless it's
already valid.

Reviewed by: kib
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D14263