UBIFS: return sensible error codes

When mounting/re-mounting, UBIFS returns EINVAL even if the ENOSPC
or EROFS codes are are much better, just because we have not found
references to ENOSPC/EROFS in mount (2) man pages. This patch
changes this behaviour and makes UBIFS return real error code,
because:

1. It is just less confusing and more logical
2. mount is not described in SuSv3, so it seems to be not really
   well-standartized
3. we do not cover all cases, and any random undocumented in man
   pages error code may be returned anyway

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/master.c

@@ -354,7 +354,7 @@ int ubifs_write_master(struct ubifs_info *c)
 	int err, lnum, offs, len;
 
 	if (c->ro_media)
-		return -EINVAL;
+		return -EROFS;
 
 	lnum = UBIFS_MST_LNUM;
 	offs = c->mst_offs + c->mst_node_alsz;

fs/ubifs/super.c

@@ -1085,12 +1085,7 @@ static int check_free_space(struct ubifs_info *c)
 		ubifs_err("insufficient free space to mount in read/write mode");
 		dbg_dump_budg(c);
 		dbg_dump_lprops(c);
-		/*
-		 * We return %-EINVAL instead of %-ENOSPC because it seems to
-		 * be the closest error code mentioned in the mount function
-		 * documentation.
-		 */
-		return -EINVAL;
+		return -ENOSPC;
 	}
 	return 0;
 }
@@ -1790,15 +1785,15 @@ static int ubifs_remount_fs(struct super_block *sb, int *flags, char *data)
 	if ((sb->s_flags & MS_RDONLY) && !(*flags & MS_RDONLY)) {
 		if (c->ro_media) {
 			ubifs_msg("cannot re-mount due to prior errors");
-			return -EINVAL;
+			return -EROFS;
 		}
 		err = ubifs_remount_rw(c);
 		if (err)
 			return err;
 	} else if (!(sb->s_flags & MS_RDONLY) && (*flags & MS_RDONLY)) {
 		if (c->ro_media) {
 			ubifs_msg("cannot re-mount due to prior errors");
-			return -EINVAL;
+			return -EROFS;
 		}
 		ubifs_remount_ro(c);
 	}