CVE-2024-42161 bpf: Avoid uninitialized value in BPF_CORE_READ_BITFIELD

In the Linux kernel, the following vulnerability has been resolved:

bpf: Avoid uninitialized value in BPF_CORE_READ_BITFIELD

[Changes from V1:

• Use a default branch in the switch statement to initialize `val’.]

GCC warns that `val’ may be used uninitialized in the
BPF_CRE_READ_BITFIELD macro, defined in bpf_core_read.h as:

[...]
unsigned long long val;						      \
[...]								      \
switch (__CORE_RELO(s, field, BYTE_SIZE)) {			      \
case 1: val = *(const unsigned char *)p; break;			      \
case 2: val = *(const unsigned short *)p; break;		      \
case 4: val = *(const unsigned int *)p; break;			      \
case 8: val = *(const unsigned long long *)p; break;		      \
    }       							      \
[...]
val;								      \
}								      \

This patch adds a default entry in the switch statement that sets
`val’ to zero in order to avoid the warning, and random values to be
used in case __builtin_preserve_field_info returns unexpected values
for BPF_FIELD_BYTE_SIZE.

Tested in bpf-next master.
No regressions.