contiguous block for future fragment expansion doesn't need to
UFS_WAPBL_REGISTER_DEALLOCATION() or ffs_blkfree(); the free blocks
are now immediatelly available for use by the expanding file in further i/o
primary driver is safe removal of the deallocation registration and
hence failure point, but this also fixes degenerate case for wapbl,
and similar also for discard - if the file would be actually expanded
before wapbl commit, or before discard queue would be processed,
the filesystem would not yet see the contiguous free blocks, and
would be forced to allocate another fragment elsewhere
9a0d9b14e1