misc: Remove VLA from uid_sys_stats.c

get_full_task_comm() uses a stack VLA in theory but it
isn't really a thing because the size is calculated immediately
before and is constant. I wish the compiler was smart enough
to realize that, but it isn't.

Change-Id: Ic0dedbad933046c920626aeabeed5e1acd885752
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>