fs/binfmt_elf.c: fix ->start_code calculation

Only executable segments should be accounted to ->start_code just like
they do to ->end_code (correctly).

Link: http://lkml.kernel.org/r/20191208171410.GB19716@avx2


Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>