writeback: merge try_to_writeback_inodes_sb_nr() into caller

Since commit 925a6efb ("Btrfs: stop using
try_to_writeback_inodes_sb_nr to flush delalloc") this function hasn't
been used outside so stop exporting it.

In addition we merge it into try_to_writeback_inodes_sb() which is the
only caller.  Also change return type of try_to_writeback_inodes_sb to
void as the only user ext4 doesn't care.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Rakesh Pandit <rakesh@tuxera.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>