f2fs: fix block_ops trace point

block operations is used to flush all dirty node and dentry blocks in
the page cache and suspend ordinary writing activities, however, there
are some facts such like cp error or mount read-only etc which lead to
block operations can't be invoked. Current trace point print block_ops
start premature even if block_ops doesn't have opportunity to execute.
This patch fix it by move block_ops trace point just before block_ops.

Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>