libbpf: Remove mode check in libbpf_set_strict_mode()

libbpf_set_strict_mode() checks that the passed mode doesn't contain
extra bits for LIBBPF_STRICT_* flags that don't exist yet.

It makes it difficult for applications to disable some strict flags as
something like "LIBBPF_STRICT_ALL & ~LIBBPF_STRICT_MAP_DEFINITIONS"
is rejected by this check and they have to use a rather complicated
formula to calculate it.[0]

One possibility is to change LIBBPF_STRICT_ALL to only contain the bits
of all existing LIBBPF_STRICT_* flags instead of 0xffffffff. However
it's not possible because the idea is that applications compiled against
older libbpf_legacy.h would still be opting into latest
LIBBPF_STRICT_ALL features.[1]

The other possibility is to remove that check so something like
"LIBBPF_STRICT_ALL & ~LIBBPF_STRICT_MAP_DEFINITIONS" is allowed. It's
what this commit does.

[0]: https://lore.kernel.org/bpf/20220204220435.301896-1-mauricio@kinvolk.io/
[1]: https://lore.kernel.org/bpf/CAEf4BzaTWa9fELJLh+bxnOb0P1EMQmaRbJVG0L+nXZdy0b8G3Q@mail.gmail.com/

Fixes: 93b8952d ("libbpf: deprecate legacy BPF map definitions")
Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220207145052.124421-2-mauricio@kinvolk.io

tools/lib/bpf/libbpf.c

@@ -156,14 +156,6 @@ enum libbpf_strict_mode libbpf_mode = LIBBPF_STRICT_NONE;
 
 int libbpf_set_strict_mode(enum libbpf_strict_mode mode)
 {
-	/* __LIBBPF_STRICT_LAST is the last power-of-2 value used + 1, so to
-	 * get all possible values we compensate last +1, and then (2*x - 1)
-	 * to get the bit mask
-	 */
-	if (mode != LIBBPF_STRICT_ALL
-	    && (mode & ~((__LIBBPF_STRICT_LAST - 1) * 2 - 1)))
-		return errno = EINVAL, -EINVAL;
-
 	libbpf_mode = mode;
 	return 0;
 }