cifsd: Pass string length parameter to match_pattern()

When iterating through a directory, a file's name may not be
null-terminated (depending on the underlying filesystem implementation).

Modify match_pattern to take the string's length into account when matching
it against the request pattern.

Signed-off-by: Marios Makassikis <mmakassikis@freebox.fr>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>