Revert "Revert "exec: move S_ISREG() check earlier""

This reverts commit 29298d15 as Kees has
submitted something upstream to fix this properly.

Cc: Kees Cook <keescook@google.com>
Cc: Marc Zyngier <mzyngier@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: Id3220d4220e77ee6cbabc4a0ccae06acef726f0e