vfs: verify source area in vfs_dedupe_file_range_one()

Call remap_verify_area() on the source file as well as the destination.

When called from vfs_dedupe_file_range() the check as already been
performed, but not so if called from layered fs (overlayfs, etc...)

Could ommit the redundant check in vfs_dedupe_file_range(), but leave for
now to get error early (for fear of breaking backward compatibility).

This call shouldn't be performance sensitive.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>