Merge branch 'add-fd_array_cnt-attribute-for-bpf_prog_load'

Anton Protopopov says:

====================
Add fd_array_cnt attribute for BPF_PROG_LOAD

Add a new attribute to the bpf(BPF_PROG_LOAD) system call. If this
new attribute is non-zero, then the fd_array is considered to be a
continuous array of the fd_array_cnt length and to contain only
proper map file descriptors or btf file descriptors.

This change allows maps (and btfs), which aren't referenced directly
by a BPF program, to be bound to the program _and_ also to be present
during the program verification (so BPF_PROG_BIND_MAP is not enough
for this use case).

The primary reason for this change is that it is a prerequisite for
adding "instruction set" maps, which are both non-referenced by the
program and must be present during the program verification.

The first five commits add the new functionality, the sixth adds
corresponding self-tests, and the last one is a small additional fix.

v1 -> v2:
  * rewrite the add_fd_from_fd_array() function (Eduard)
  * a few cleanups in selftests (Eduard)

v2 -> v3:
  * various renamings (Alexei)
  * "0 is not special" (Alexei, Andrii)
  * do not alloc memory on fd_array init (Alexei)
  * fix leaking maps for error path (Hou Tao)
  * use libbpf helpers vs. raw syscalls (Andrii)
  * add comments on __btf_get_by_fd/__bpf_map_get (Alexei)
  * remove extra code (Alexei)

v3 -> v4:
  * simplify error path when parsing fd_array
  * libbpf: pass fd_array_cnt only in prog_load (Alexei)
  * selftests patch contained extra code (Alexei)
  * renames, fix comments (Alexei)

v4 -> v5:
  * Add btfs to env->used_btfs (Andrii)
  * Fix an integer overflow (Andrii)
  * A set of cleanups for selftests (Andrii)
====================

Link: https://patch.msgid.link/20241213130934.1087929-1-aspsk@isovalent.com
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>