libthr malloc: support recursion on thr_malloc_umtx.

libthr malloc: support recursion on thr_malloc_umtx.

One possible way the recursion can happen is during fork: suppose
that fork is called from early code that did not triggered
jemalloc(3) initialization yet. Then we lock thr_malloc lock, and
call malloc_prefork() that might require initialization of jemalloc
pthread_mutexes, calling into libthr malloc. It is safe to allow
recursion for this occurence.

PR:	252579
Reported by:	Vasily Postnicov <shamaz.mazum@gmail.com>
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation