Fix up a misuse that the filename pointer isn't always valid in
the ring buffer, and we should copy the content instead.

Fixes: 0c5e36db ("f2fs: trace f2fs_lookup")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>