trace: Use vmalloc for allocating trace comm and tgid caches

The size of the allocations depends on saved_cmdlines_size. Use vmalloc
to make sure the allocations succeed.

Bug: 62669736

Change-Id: Ic8943b5ac3c5afe8a8c0f5b58974dcc6880b0d5c
Signed-off-by: Joel Fernandes <joelaf@google.com>