System rebooted suddenly when cifsd tried to access unmounted volume.

Issue

(1) Installing RHEL6 (2.6.32-431.3.1)
(2) Installing Acronis Backup product and run backup to Windows Storage Server 2008 (WSS2K8) manually.
(3) After finishing backup, they removed the cable between switch <=> WSS2K8.

 [RHEL] --- [Switch] -x- [WSS2K8]

(4) Following error messages are logged in /var/log/messages continuously.

 Jan 20 20:00:22 localhost kernel: CIFS VFS: Unexpected lookup error -112

(5) Next morning, the system rebooted with vmcore.

The error of CIFS were recorded just before in /var/log/messages.

        Jan 21 04:57:02 localhost kernel: CIFS VFS: Unexpected lookup error -112
        Jan 21 04:58:12 localhost kernel: CIFS VFS: Unexpected lookup error -112
        Jan 21 04:59:42 localhost kernel: CIFS VFS: Unexpected lookup error -112
        Jan 21 05:01:02 localhost kernel: CIFS VFS: Unexpected lookup error -112
        Jan 21 05:02:32 localhost kernel: CIFS VFS: Unexpected lookup error -112

vmcore shows followings.

        BUG: unable to handle kernel NULL pointer dereference at 0000000000000038
        IP: [<ffffffff81449a7b>] sock_recvmsg+0xeb/0x160
        PGD 212ab6067 PUD 212acb067 PMD 0 
        Oops: 0000 [#1] SMP 
        last sysfs file: /sys/devices/system/cpu/online
        CPU 1 
        Modules linked in: snumbd26(P)(U) des_generic ecb md4 cifs snapapi26(P)(U) nls_utf8 fuse autofs4 ipv6 uinput microcode iTCO_wdt iTCO_vendor_support dcdbas sb_edac edac_core power_meter sg lpc_ich mfd_core shpchp tg3 ptp pps_core ext4 jbd2 mbcache sr_mod cdrom sd_mod crc_t10dif wmi ahci megaraid_sas dm_mirror dm_region_hash dm_log dm_mod [last unloaded: edd]

        Pid: 7204, comm: cifsd Tainted: P           ---------------    2.6.32-431.3.1.el6.x86_64 #1 Dell Inc. PowerEdge R320/0R5KP9
        RIP: 0010:[<ffffffff81449a7b>]  [<ffffffff81449a7b>] sock_recvmsg+0xeb/0x160

And some modules were considered to be the things of Acronis.

        snumbd26(P)(U), snapapi26(P)(U)