Fix warning about sleeping with non-sleepable lock when allocating
"current" from linux_cdev_pager_populate() in the LinuxKPI:

Backtrace:
witness_debugger()
witness_warn()
uma_zalloc_arg()
malloc()
linux_alloc_current()
linux_cdev_pager_populate()
vm_fault()
vm_fault_trap()
trap_pfault()
trap()
calltrap()

Suggested by: avg@
MFC after: 1 week
Sponsored by: Mellanox Technologies