cifs: fix warning when destroy 'cifs_io_request_pool'

There's a issue as follows:
WARNING: CPU: 1 PID: 27826 at mm/slub.c:4698 free_large_kmalloc+0xac/0xe0
RIP: 0010:free_large_kmalloc+0xac/0xe0
Call Trace:
 <TASK>
 ? __warn+0xea/0x330
 mempool_destroy+0x13f/0x1d0
 init_cifs+0xa50/0xff0 [cifs]
 do_one_initcall+0xdc/0x550
 do_init_module+0x22d/0x6b0
 load_module+0x4e96/0x5ff0
 init_module_from_file+0xcd/0x130
 idempotent_init_module+0x330/0x620
 __x64_sys_finit_module+0xb3/0x110
 do_syscall_64+0xc1/0x1d0
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Obviously, 'cifs_io_request_pool' is not created by mempool_create().
So just use mempool_exit() to revert 'cifs_io_request_pool'.

Fixes: edea94a6 ("cifs: Add mempools for cifs_io_request and cifs_io_subrequest structs")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>