xfs: Convert double locking of MMAPLOCK to use VFS helpers
Convert places in XFS that take MMAPLOCK for two inodes to use helper
VFS provides for it (filemap_invalidate_down_write_two()). Note that
this changes lock ordering for MMAPLOCK from inode number based ordering
to pointer based ordering VFS generally uses.
CC: "Darrick J. Wong" <djwong@kernel.org> Reviewed-by: Darrick J. Wong <djwong@kernel.org> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jan Kara <jack@suse.cz>