lsm: fix typo in security_file_lock() comment header

In the description of function definition security_file_lock(),
the line "@cmd: fnctl command" has a typo where "fnctl" should be
"fcntl". This patch fixes the typo.

Signed-off-by: Pairman Guo <pairmanxlr@gmail.com>
[PM: commit message cleanup]
Signed-off-by: Paul Moore <paul@paul-moore.com>