cifs: release cifs root_cred after exit_cifs

cifs: release cifs root_cred after exit_cifs

memory leak was found by kmemleak. exit_cifs_spnego
should be called before cifs module removed, or
cifs root_cred will not be released.

kmemleak report:
unreferenced object 0xffff880070a3ce40 (size 192):
  backtrace:
     kmemleak_alloc+0x4a/0xa0
     kmem_cache_alloc+0xc7/0x1d0
     prepare_kernel_cred+0x20/0x120
     init_cifs_spnego+0x2d/0x170 [cifs]
     0xffffffffc07801f3
     do_one_initcall+0x51/0x1b0
     do_init_module+0x60/0x1fd
     load_module+0x161e/0x1b60
     SYSC_finit_module+0xa9/0x100
     SyS_finit_module+0xe/0x10

Signed-off-by: Shu Wang <shuwang@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
CC: Stable <stable@vger.kernel.org>