xfs: make mem_to_page available outside of xfs_buf.c

Rename the function to kmem_to_page and move it to kmem.h together
with our kmem_large allocator that may either return kmalloced or
vmalloc pages.

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