btrfs: consolidate reclaim readiness checks in btrfs_should_reclaim()

btrfs: consolidate reclaim readiness checks in btrfs_should_reclaim()

Move the filesystem state validation from btrfs_reclaim_bgs_work() into
btrfs_should_reclaim() to centralize the reclaim eligibility logic.
Since it is the only caller of btrfs_should_reclaim(), there's no
functional change.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Sun YangKai <sunk67188@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>