vfs: export remap and write check helpers

Export these functions so that the next patch can use them to check the
file ranges being passed to the XFS_IOC_EXCHANGE_RANGE operation.

Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>