LU-12400 libcfs: save_stack_trace_tsk if ARCH_STACKWALK

LU-12400 libcfs: save_stack_trace_tsk if ARCH_STACKWALK

Along with CONFIG_ARCH_STACKWALK save_stack_trace_tsk is not
directly available. Try using symbol_get() to acquire it.

Linux-commit: 214d8ca6ee854f696f75e75511fe66b409e656db

Test-Parameters: trivial
Cray-bug-id: LUS-7600
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: I923b718eadc6c58fa2676a6d2fbd48523c615f62
Reviewed-on: https://review.whamcloud.com/35239
Reviewed-by: Chris Horn <hornc@cray.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>