bpf: make list_for_each_entry portable

[Changes from V1:
- The __compat_break has been abandoned in favor of
  a more readable can_loop macro that can be used anywhere, including
  loop conditions.]

The macro list_for_each_entry is defined in bpf_arena_list.h as
follows:

  #define list_for_each_entry(pos, head, member)				\
	for (void * ___tmp = (pos = list_entry_safe((head)->first,		\
						    typeof(*(pos)), member),	\
			      (void *)0);					\
	     pos && ({ ___tmp = (void *)pos->member.next; 1; });		\
	     cond_break,							\
	     pos = list_entry_safe((void __arena *)___tmp, typeof(*(pos)), member))

The macro cond_break, in turn, expands to a statement expression that
contains a `break' statement.  Compound statement expressions, and the
subsequent ability of placing statements in the header of a `for'
loop, are GNU extensions.

Unfortunately, clang implements this GNU extension differently than
GCC:

- In GCC the `break' statement is bound to the containing "breakable"
  context in which the defining `for' appears.  If there is no such
  context, GCC emits a warning: break statement without enclosing `for'
  o `switch' statement.

- In clang the `break' statement is bound to the defining `for'.  If
  the defining `for' is itself inside some breakable construct, then
  clang emits a -Wgcc-compat warning.

This patch adds a new macro can_loop to bpf_experimental, that
implements the same logic than cond_break but evaluates to a boolean
expression.  The patch also changes all the current instances of usage
of cond_break withing the header of loop accordingly.

Tested in bpf-next master.
No regressions.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Cc: david.faust@oracle.com
Cc: cupertino.miranda@oracle.com
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Link: https://lore.kernel.org/r/20240511212243.23477-1-jose.marchesi@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>