btrfs: harden parsing of compression mount options

Btrfs happily but incorrectly accepts the `-o compress=zlib+foo` and similar
options with any random suffix.

Fix that by explicitly checking the end of the strings.

Signed-off-by: Daniel Vacek <neelx@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>