vfs: dedupe should return EPERM if permission is not granted

Right now we return EINVAL if a process does not have permission to dedupe a
file. This was an oversight on my part. EPERM gives a true description of
the nature of our error, and EINVAL is already used for the case that the
filesystem does not support dedupe.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Acked-by: David Sterba <dsterba@suse.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>