[XFS] Fix nused counter. It's currently getting set to -1 rather than

getting decremented by 1.  Since nused never reaches 0, the "if
(!free->hdr.nused)" check in xfs_dir2_leafn_remove() fails every time and
xfs_dir2_shrink_inode() doesn't get called when it should.  This causes
extra blocks to be left on an empty directory and the directory in unable
to be converted back to inline extent mode.

SGI-PV: 951958
SGI-Modid: xfs-linux-melb:xfs-kern:211382a

Signed-off-by: Mandy Kirkconnell <alkirkco@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>