kernel/mem/lib/mem.c: fixed tst_brkm with TINFO

tst_brkm() doesn't support TINFO, and everything in alloc_mem()
makes me feel that if the mlock(s, length) fails, we should exit
with TBROK (as we do for the mmap and madvise parts).

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>