btrfs: use appropriate replacements for __sb_{start,end}_write calls

Commit a53f4f8e ("btrfs: Don't call btrfs_start_transaction() on
frozen fs to avoid deadlock.") started using internal calls and we
replace them with more suitable ones.

Signed-off-by: Rakesh Pandit <rakesh@tuxera.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>