I've done kernel implementation for 20 years, and this question is absolutely terrible -- unless the answer that one is looking for is an intelligent explanation of why this is such a terrible question. It's an awful question because the kernel may not be involved at all on a user-level call to malloc() (e.g., a caching allocator like libumem or a traditional allocator that needn't extend the break for a given allocation), may be involved in a minor capacity if the break must be extended (on most systems, a system call and some VM interaction), or could be involved substantially if the allocator is a mapping allocator and page faults are induced (text and data) or scheduling events or anything else that necessitates kernel involvement.
For whatever it's worth, when I have historically interviewed university hires for kernel positions, the question I ask is much simpler; namely, what does "* ((int *)NULL)" do? Sadly, most university students -- even ones who have done very well in their university computer science courses -- don't get this right. And it's sad because it's not really the question, but rather just the segue to the actual question: when a candidate correctly answers that NULL is dereferenced and the program crashes[1], I ask them to write the program (statement, really) in an assembly of their choosing (most graduates have MIPS or SPARC on the resume) and I then ask for everything that happens between the execution of that instruction and the resulting core dump and return of control to the shell. With a qualified candidate, this could easily be a multi-hour jaunt through microprocessor architecture and operating systems implementation.
All of that said: I don't really do that anymore, but rather look to a candidate's open source works. The last kernel engineer I hired had done open source work that was so impressive that my interview consisted only of me determining that he had done the work himself -- which took all of 30 seconds or so. (He had, I hired him and he's been an absolutely terrific engineer.) Easiest interview ever!
[1] Except on AIX and any other asinine system that maps NULL.