btrfs: shorten critical section in btrfs_preempt_reclaim_metadata_space()

btrfs: shorten critical section in btrfs_preempt_reclaim_metadata_space()

We are doing a lot of small calculations and assignments while holding the
space_info's spinlock, which is a heavily used lock for space reservation
and flushing. There's no point in holding the lock for so long when all we
want is to call need_preemptive_reclaim() and get a consistent value for a
couple of counters from the space_info. Instead, grab the counters into
local variables, release the lock and then use the local variables.

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>