bpf, arm64: Force 8-byte alignment for JIT buffer to prevent atomic tearing

struct bpf_plt contains a u64 target field. Currently, the BPF JIT
allocator requests an alignment of 4 bytes (sizeof(u32)) for the JIT
buffer.

Because the base address of the JIT buffer can be 4-byte aligned (e.g.,
ending in 0x4 or 0xc), the relative padding logic in build_plt() fails
to ensure that target lands on an 8-byte boundary.

This leads to two issues:
1. UBSAN reports misaligned-access warnings when dereferencing the
structure.
2. More critically, target is updated concurrently via WRITE_ONCE() in
bpf_arch_text_poke() while the JIT'd code executes ldr. On arm64,
64-bit loads/stores are only guaranteed to be single-copy atomic if
they are 64-bit aligned. A misaligned target risks a torn read,
causing the JIT to jump to a corrupted address.

Fix this by increasing the allocation alignment requirement to 8 bytes
(sizeof(u64)) in bpf_jit_binary_pack_alloc(). This anchors the base of
the JIT buffer to an 8-byte boundary, allowing the relative padding math
in build_plt() to correctly align the target field.

Fixes: b2ad54e1533e ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
Signed-off-by: Fuad Tabba <tabba@google.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20260226075525.233321-1-tabba@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>