libbpf: Fix error handling in bpf_map__reuse_fd()

libbpf: Fix error handling in bpf_map__reuse_fd()

[ Upstream commit d1b4574a4b86565325ef2e545eda8dfc9aa07c60 ]

bpf_map__reuse_fd() was calling close() in the error path before returning
an error value based on errno. However, close can change errno, so that can
lead to potentially misleading error messages. Instead, explicitly store
errno in the err variable before each goto.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/157269297769.394725.12634985106772698611.stgit@toke.dk
Signed-off-by: Sasha Levin <sashal@kernel.org>