Merge branch 'queue_stack_maps'

Mauricio Vasquez says:

====================
In some applications this is needed have a pool of free elements, for
example the list of free L4 ports in a SNAT.  None of the current maps allow
to do it as it is not possible to get any element without having they key
it is associated to, even if it were possible, the lack of locking mecanishms in
eBPF would do it almost impossible to be implemented without data races.

This patchset implements two new kind of eBPF maps: queue and stack.
Those maps provide to eBPF programs the peek, push and pop operations, and for
userspace applications a new bpf_map_lookup_and_delete_elem() is added.

Signed-off-by: Mauricio Vasquez B <mauricio.vasquez@polito.it>

v2 -> v3:
 - Remove "almost dead code" in syscall.c
 - Remove unnecessary copy_from_user in bpf_map_lookup_and_delete_elem
 - Rebase

v1 -> v2:
 - Put ARG_PTR_TO_UNINIT_MAP_VALUE logic into a separated patch
 - Fix missing __this_cpu_dec & preempt_enable calls in kernel/bpf/syscall.c

RFC v4 -> v1:
 - Remove roundup to power of 2 in memory allocation
 - Remove count and use a free slot to check if queue/stack is empty
 - Use if + assigment for wrapping indexes
 - Fix some minor style issues
 - Squash two patches together

RFC v3 -> RFC v4:
 - Revert renaming of kernel/bpf/stackmap.c
 - Remove restriction on value size
 - Remove len arguments from peek/pop helpers
 - Add new ARG_PTR_TO_UNINIT_MAP_VALUE

RFC v2 -> RFC v3:
 - Return elements by value instead that by reference
 - Implement queue/stack base on array and head + tail indexes
 - Rename stack trace related files to avoid confusion and conflicts

RFC v1 -> RFC v2:
 - Create two separate maps instead of single one + flags
 - Implement bpf_map_lookup_and_delete syscall
 - Support peek operation
 - Define replacement policy through flags in the update() method
 - Add eBPF side tests
====================

Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>