Minor cleanup in __kmp_atfork_child()

This change removes the unnecessary lock operation on __kmp_initz_lock inside
the __kmp_atfork_child() function for Linux; the lock variable is initialized
in the same function later.

Patch by Hansang Bae

Differential Revision: https://reviews.llvm.org/D44949


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@328900 91177308-0d34-0410-b5e6-96231b3b80d8