Similar to rmdir and remove - avoids a potential transaction reservation
overrun.

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30900a
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>