cifs: properly handle case where CIFSGetSrvInodeNumber fails

...if it does then we pass a pointer to an unintialized variable for
the inode number to cifs_new_inode. Have it pass a NULL pointer instead.

Also tweak the function prototypes to reduce the amount of casting.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>