kexec-tools: Check callback first in kexec_iomem_for_each_line()

In the function kexec_iomem_for_each_line(), it is better to
check the callback first, it can return directly if the callback
is NULL.

Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Simon Horman <horms@verge.net.au>