btrfs: allow setting zlib compression level via :9

This is bikeshedding, but it seems people are drastically more likely to
understand "zlib:9" as compression level rather than an algorithm
version compared to "zlib9".

Based on feedback on the mailinglist, the ":9" will be the only accepted
syntax. The level must be a single digit. Unrecognized format will
result to the default, for forward compatibility in a similar way the
compression algorithm specifier was relaxed in commit
a7164fa4e055daf6368c ("btrfs: prepare for extensions in compression
options").

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Reviewed-by: David Sterba <dsterba@suse.com>
[ tighten the accepted format ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/compression.c

@@ -1111,8 +1111,9 @@ unsigned int btrfs_compress_str2level(const char *str)
 	if (strncmp(str, "zlib", 4) != 0)
 		return 0;
 
-	if ('1' <= str[4] && str[4] <= '9' )
-		return str[4] - '0';
+	/* Accepted form: zlib:1 up to zlib:9 and nothing left after the number */
+	if (str[4] == ':' && '1' <= str[5] && str[5] <= '9' && str[6] == 0)
+		return str[5] - '0';
 
 	return 0;
 }

fs/btrfs/super.c

@@ -1261,7 +1261,7 @@ static int btrfs_show_options(struct seq_file *seq, struct dentry *dentry)
 		else
 			seq_printf(seq, ",compress=%s", compress_type);
 		if (info->compress_level)
-			seq_printf(seq, "%d", info->compress_level);
+			seq_printf(seq, ":%d", info->compress_level);
 	}
 	if (btrfs_test_opt(info, NOSSD))
 		seq_puts(seq, ",nossd");