test/common: Reset IFS within print_backtrace()

In case IFS was modified in the environment where print_backtrace()
was called, it could impact the cmd substitution used for seq'ing
the main FUNCNAME loop. Example:

========== Backtrace start: ==========

/home/vagrant/spdk_repo/spdk/test/common/autotest_common.sh: line 980: 1
2
3
4: syntax error in expression (error token is "2
3
4")

Avoid that by resetting the IFS to its default value, but also,
replace said $() with arithmetic evaluation to avoid it altogether.

Signed-off-by: Michal Berger <michalx.berger@intel.com>
Change-Id: I4ab098319dfae3a5356eb4fe0dbf9f4af2d2eea6
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/8013


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>