Simplify the futex lock loop: no need for nested while

Simplify the futex lock loop: no need for nested while

Once we enter the inner loop, we never exit it except to return from
the lockInternal() function, so the rest is never executed again.

As a consequence of this, we won't try to fastTryLock() twice per
mutex. Therefore, for a non-recursive mutex, if lockInternal() is
entered, we'll definitely need to use futexes.

Change-Id: Ice617ed27449c1fbdc112a159a86cd0660125e13
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>