btrfs: use enum for btrfs_block_rsv::type

The number of block group reserve types BTRFS_BLOCK_RSV_* is small and
fits to u8 and there's enough left in case we want to add more.
For type safety use the enum but make it 8 bits in the structure to save
space.

The structure size is now 48 on release build, making a slight
improvement in structures where it's embedded, like btrfs_fs_info or
btrfs_inode.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>