selftests/bpf: Fix fill_link_info selftest on powerpc

selftests/bpf: Fix fill_link_info selftest on powerpc

With CONFIG_KPROBES_ON_FTRACE enabled on powerpc, ftrace_location_range
returns ftrace location for bpf_fentry_test1 at offset of 4 bytes from
function entry. This is because branch to _mcount function is at offset
of 4 bytes in function profile sequence.

To fix this, add entry_offset of 4 bytes while verifying the address for
kprobe entry address of bpf_fentry_test1 in verify_perf_link_info in
selftest, when CONFIG_KPROBES_ON_FTRACE is enabled.

Disassemble of bpf_fentry_test1:

c000000000e4b080 <bpf_fentry_test1>:
c000000000e4b080:       a6 02 08 7c     mflr    r0
c000000000e4b084:       b9 e2 22 4b     bl      c00000000007933c <_mcount>
c000000000e4b088:       01 00 63 38     addi    r3,r3,1
c000000000e4b08c:       b4 07 63 7c     extsw   r3,r3
c000000000e4b090:       20 00 80 4e     blr

When CONFIG_PPC_FTRACE_OUT_OF_LINE [1] is enabled, these function profile
sequence is moved out of line with an unconditional branch at offset 0.
So, the test works without altering the offset for
'CONFIG_KPROBES_ON_FTRACE && CONFIG_PPC_FTRACE_OUT_OF_LINE' case.

Disassemble of bpf_fentry_test1:

c000000000f95190 <bpf_fentry_test1>:
c000000000f95190:       00 00 00 60     nop
c000000000f95194:       01 00 63 38     addi    r3,r3,1
c000000000f95198:       b4 07 63 7c     extsw   r3,r3
c000000000f9519c:       20 00 80 4e     blr

[1] https://lore.kernel.org/all/20241030070850.1361304-13-hbathini@linux.ibm.com/

Fixes: 23cf7aa539dc ("selftests/bpf: Add selftest for fill_link_info")
Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241209065720.234344-1-skb99@linux.ibm.com