do_execve(): lock vnode when needed.

Code after exec_fail_dealloc label expects that the image vnode is
locked if present. When copyout() of the strings or auxv vectors fails,
goto to the error handling did not relocked the vnode as required.

The copyout() can be made failing e.g. by creating an ELF image with
PT_GNU_STACK segment disabling the write.

Reported by: Jonathan Stuart <n0t.jcs@gmail.com> (found by fuzzing)
Sponsored by: The FreeBSD Foundation
MFC after: 3 days