[lsan] Fix deadlock in dl_iterate_phdr.

Summary:
Do not grab the allocator lock before calling dl_iterate_phdr. This may
cause a lock order inversion with (valid) user code that uses malloc
inside a dl_iterate_phdr callback.

Reviewers: vitalybuka, hctim

Subscribers: jfb, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D67738

llvm-svn: 372348