btrfs: reduce block group critical section in btrfs_add_reserved_bytes()

btrfs: reduce block group critical section in btrfs_add_reserved_bytes()

We are doing some things inside the block group's critical section that
are relevant only to the space_info: updating the space_info counters
bytes_reserved and bytes_may_use as well as trying to grant tickets
(calling btrfs_try_granting_tickets()), and this later can take some
time. So move all those updates to outside the block group's critical
section and still inside the space_info's critical section. Like this
we keep the block group's critical section only for block group updates
and can help reduce contention on a block group's lock.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>