xfs: always take XFS_MMAPLOCK shared in xfs_dax_read_fault

After the previous refactoring, xfs_dax_fault is now never used for write
faults, so don't bother with the xfs_ilock_for_write_fault logic to
protect against writes when remapping is in progress.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>