[XFS] allow xfs_args_allocate to fail

Switch xfs_args_allocate to kzalloc and handle failures.

SGI-PV: 981951
SGI-Modid: xfs-linux-melb:xfs-kern:31195a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>