btrfs: don't include linux/rwlock_types.h directly

The header clearly states that it does not want to be included directly,
only via linux/spinlock_types.h. Drop this as we can simply use the
spinlock.h which is already included.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>