Btrfs: use helpers for last_trans_log_full_commit instead of opencode

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

diff --cc fs/btrfs/extent-tree.c
Simple merge

diff --cc fs/btrfs/inode.c
Simple merge

diff --cc fs/btrfs/tree-log.c
Simple merge

diff --cc fs/btrfs/tree-log.h
Simple merge