libbpf: Fix using invalidated memory in bpf_linker

libbpf: Fix using invalidated memory in bpf_linker

[ Upstream commit 593835377f24ca1bb98008ec1dc3baefe491ad6e ]

add_dst_sec() can invalidate bpf_linker's section index making
dst_symtab pointer pointing into unallocated memory. Reinitialize
dst_symtab pointer on each iteration to make sure it's always valid.

Fixes: faf6ed321cf6 ("libbpf: Add BPF static linker APIs")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211124002325.1737739-7-andrii@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>