xfs: allow xfs_lock_two_inodes to take different EXCL/SHARED modes

Refactor xfs_lock_two_inodes to take separate locking modes for each
inode.  Specifically, this enables us to take a SHARED lock on one inode
and an EXCL lock on the other.  The lock class (MMAPLOCK/ILOCK) must be
the same for each inode.

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