xfs: only update mount/resv fields on success in __xfs_ag_resv_init

xfs: only update mount/resv fields on success in __xfs_ag_resv_init

Source kernel commit: 4dfa2b84118fd6c95202ae87e62adf5000ccd4d0

Try to reserve the blocks first and only then update the fields in
or hanging off the mount structure.  This way we can call __xfs_ag_resv_init
again after a previous failure.

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