bpf: Migrate dynptr file to kmalloc_nolock

bpf: Migrate dynptr file to kmalloc_nolock

Replace bpf_mem_alloc/bpf_mem_free with kmalloc_nolock/kfree_nolock for
bpf_dynptr_file_impl, continuing the migration away from bpf_mem_alloc
now that kmalloc can be used from NMI context.

freader_cleanup() runs before kfree_nolock() while the dynptr still
holds exclusive access, so plain kfree_nolock() is safe — no concurrent
readers can access the object.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260330-kmalloc_special-v2-2-c90403f92ff0@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>