f2fs: fix the msg data type

Data type of msg in f2fs_write_checkpoint trace should
be const char * instead of char *.

Signed-off-by: Mukesh Ojha <quic_mojha@quicinc.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>