[XFS] use KM_MAYFAIL in xfs_mountfs

Use KM_MAYFAIL for the m_perag allocation, we can deal with the error
easily and blocking forever during mount is not a good idea either.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31837a

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