bpf: sockmap: Remove preempt_disable in sock_map_sk_acquire

Disabling preemption in sock_map_sk_acquire conflicts with GFP_ATOMIC
allocation later in sk_psock_init_link on PREEMPT_RT kernels, since
GFP_ATOMIC might sleep on RT (see bpf: Make BPF and PREEMPT_RT co-exist
patchset notes for details).

This causes calling bpf_map_update_elem on BPF_MAP_TYPE_SOCKMAP maps to
BUG (sleeping function called from invalid context) on RT kernels.

preempt_disable was introduced together with lock_sk and rcu_read_lock
in commit 99ba2b5a

 ("bpf: sockhash, disallow bpf_tcp_close and update
in parallel"), probably to match disabled migration of BPF programs, and
is no longer necessary.

Remove preempt_disable to fix BUG in sock_map_update_common on RT.

Signed-off-by: Tomas Glozar <tglozar@redhat.com>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/all/20200224140131.461979697@linutronix.de/
Fixes: 99ba2b5a

 ("bpf: sockhash, disallow bpf_tcp_close and update in parallel")
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20230728064411.305576-1-tglozar@redhat.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>