[XFS] Prevent the incore superblock sb_fdblocks count from leaking when we

are getting ENOSPC errors on writes. When we fail to allocate space for
indirect blocks in xfs_bmapi() make sure we release the direct block
allocation before returning.

SGI-PV: 938502
SGI-Modid: xfs-linux:xfs-kern:22986a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>