btrfs: switch BTRFS_FS_* to enums

We can use simple enum for values that are not part of on-disk format:
internal filesystem states.

Reviewed-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>