Revert "exec: move S_ISREG() check earlier"

This reverts commit 633fb6ac as it
breaks execev() semantics.

Fixes: 633fb6ac ("exec: move S_ISREG() check earlier")
Cc: Kees Cook <keescook@google.com>
Reported-by: Marc Zyngier <mzyngier@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I50eb97745b1ee4f8bd5d3f1d1e6c98e30aa7c5ea