smb3: rc uninitialized in one fallocate path

Clang detected a problem with rc possibly being unitialized
(when length is zero) in a recently added fallocate code path.

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>