xfs: make xfs_*read_agf return EAGAIN to ALLOC_FLAG_TRYLOCK callers

xfs: make xfs_*read_agf return EAGAIN to ALLOC_FLAG_TRYLOCK callers

Source kernel commit: f48e2df8a877ca1c19d92cfd7e74cc5956fa84cb

Refactor xfs_read_agf and xfs_alloc_read_agf to return EAGAIN if the
caller passed TRYLOCK and we weren't able to get the lock; and change
the callers to recognize this.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>