In RHEL6, nfs server can crash within lockd from dereferencing a NULL pointer in nlmsvc_unlink_block().

Issue

• nfs server can crash within lockd from dereferencing a NULL pointer in nlmsvc_unlink_block().

PID: 5331   TASK: ffff880828096ae0  CPU: 26  COMMAND: "lockd"
 #0 [ffff880823f099f0] machine_kexec at ffffffff8103284b
 #1 [ffff880823f09a50] crash_kexec at ffffffff810ba982
 #2 [ffff880823f09b20] oops_end at ffffffff81501b00
 #3 [ffff880823f09b50] no_context at ffffffff81043bfb
 #4 [ffff880823f09ba0] __bad_area_nosemaphore at ffffffff81043e85
 #5 [ffff880823f09bf0] bad_area_nosemaphore at ffffffff81043f53
 #6 [ffff880823f09c00] __do_page_fault at ffffffff810446b1
 #7 [ffff880823f09d20] do_page_fault at ffffffff81503ade
 #8 [ffff880823f09d50] page_fault at ffffffff81500e95
    [exception RIP: nlmsvc_unlink_block+49]
    RIP: ffffffffa03ca861  RSP: ffff880823f09e00  RFLAGS: 00010246
    RAX: 0000000000000000  RBX: ffffffffa03d3d98  RCX: 000000000000ec98
    RDX: 0000000000000000  RSI: ffffffffa03cbe70  RDI: ffffffffa03d3d98
    RBP: ffff880823f09e20   R8: 0000000000017ac5   R9: 0000000000000000
    R10: 000000000000000f  R11: 000000000000000b  R12: ffffffffa03d3da0
    R13: 0000000000000000  R14: ffff880823f09e60  R15: 00000000000000bc
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
 #9 [ffff880823f09e28] nlmsvc_retry_blocked at ffffffffa03ca9f0 [lockd]
#10 [ffff880823f09e58] lockd at ffffffffa03ca236 [lockd]
#11 [ffff880823f09ee8] kthread at ffffffff81091e06
#12 [ffff880823f09f48] kernel_thread at ffffffff8100c14a