Correctly handle errors from kern_path

As a regular kernel function, kern_path() returns errors as negative
errnos, such as -ELOOP. zfsctl_snapdir_vget() must convert these into
the positive errnos used throughout the ZFS code when it returns them
to other ZFS functions so that the ZFS code properly sees them as
errors.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chris Siebenmann <cks.git01@cs.toronto.edu>
Closes #7764
Closes #7864

module/zfs/zfs_ctldir.c

@@ -1177,7 +1177,7 @@ zfsctl_snapdir_vget(struct super_block *sb, uint64_t objsetid, int gen,
 		goto out;
 
 	/* Trigger automount */
-	error = kern_path(mnt, LOOKUP_FOLLOW|LOOKUP_DIRECTORY, &path);
+	error = -kern_path(mnt, LOOKUP_FOLLOW|LOOKUP_DIRECTORY, &path);
 	if (error)
 		goto out;