f2fs: cover cp_file information with ilock

If a file is linked with other files, it should be checkpointed at every fsync
calls.
For this, we use set_cp_file() with FADVISE_CP_BIT, but previously we didn't
cover the flag by the global lock.
This patch fixes that the inode page stores this correctly.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>