The last argument to vm_object_page_clean() are now bit flags, rather than
the old true/false.

While here, have vfs_msync() only call vm_object_page_clean() with
OBJPC_SYNC if called with MNT_WAIT flags. vfs_msync() is called at unmount
time (with MNT_WAIT) and from the syncer process (formerly update).
This should make dirty mmap writebacks a little less nasty.

I have tested this a little with SOFTUPDATES enabled, but I don't normally
use it since I've been badly burned too many times.