Minor cosmetic changes for simple attribute of stacktrace_depth:

 - use min_t()
 - reduce #ifdef by moving a function
 - do not use partly capitalized function name
Signed-off-by: NAkinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>