cifsd: enclose macro variables in parenthesis

checkpatch.pl complains as the following:

CHECK: Macro argument 'fp' may be better as '(fp)' to avoid
precedence issues.

Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>