apparmor: domain: clean up duplicated parts of handle_onexec()

Regression test of AppArmor finished without any failures.

PASSED: aa_exec access attach_disconnected at_secure introspect
capabilities changeprofile onexec changehat changehat_fork
changehat_misc chdir clone coredump deleted e2e environ exec exec_qual
fchdir fd_inheritance fork i18n link link_subset mkdir mmap mount
mult_mount named_pipe namespaces net_raw open openat pipe pivot_root
posix_ipc ptrace pwrite query_label regex rename readdir rw socketpair
swap sd_flags setattr symlink syscall sysv_ipc tcp unix_fd_server
unix_socket_pathname unix_socket_abstract unix_socket_unnamed
unix_socket_autobind unlink userns xattrs xattrs_profile longpath nfs
exec_stack aa_policy_cache nnp stackonexec stackprofile
FAILED:
make: Leaving directory '/apparmor/tests/regression/apparmor'

Signed-off-by: Leesoo Ahn <lsahn@ooseel.net>
Signed-off-by: John Johansen <john.johansen@canonical.com>