With install_exec_creds updated to follow immediately after
setup_new_exec, the failure of unshare_sighand is the only
code path where exec_update_mutex is held but not explicitly
unlocked.

Update that code path to explicitly unlock exec_update_mutex.

Remove the unlocking of exec_update_mutex from free_bprm.
Reviewed-by: NKees Cook <keescook@chromium.org>
Reviewed-by: NGreg Ungerer <gerg@linux-m68k.org>
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>