selftests/bpf: Fix bpf_mod_race test failure with arm64 64KB page size

selftests/bpf: Fix bpf_mod_race test failure with arm64 64KB page size

Currently, uffd_register.range.len is set to 4096 for command
'ioctl(uffd, UFFDIO_REGISTER, &uffd_register)'. For arm64 64KB page size,
the len must be 64KB size aligned as page size alignment is required.
See fs/userfaultfd.c:validate_unaligned_range().

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250607013615.1551783-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>