perf: Track the perf mlock gift separately

The locked_vm field in struct user_struct was originally introduced by
commit 789f90fcf6b0 ("perf_counter: per user mlock gift") to provide an
mlock-like budget for perf measured against mm->pinned_vm (since 2011).

perf grants each user a 'gift' of perf_event_mlock_kb * online CPUs of
buffer pages which is charged to user->locked_vm without any RLIMIT_MEMLOCK
check.

Only pages beyond the gift are checked against RLIMIT_MEMLOCK, per-mm, via
mm->pinned_vm, which makes it different from the standard mlock() check,
which is per-mm and made against RLIMIT_MEMLOCK.

However, since this was introduced, a number of other components have
utilised this field where a shared resource needed to be limited against
RLIMIT_MEMLOCK.

The other users are currently MSG_ZEROCOPY, io_uring, AF_XDP, iommufd,
s390 KVM zPCI and most recently, secretmem in commit 97d34aa65c29
("mm/secretmem: properly account locked pages").

This field is shared between all of these, but because the gift is not
checked against RLIMIT_MEMLOCK, user->locked_vm alone can exceed it - 16
CPUs * 516 KiB is already more than the default 8 MiB - after which every
other user of the field fails unconditionally.

This was reported by a user who saw secretmem failing while running a
simultaneous perf record.

Resolve the issue by simply giving perf its own field separate from the
rest.

BPF used user->locked_vm up until commit 80ee81e0403c ("bpf: Eliminate
rlimit-based memory accounting infra for bpf maps") which landed in 5.11.

Thus also eliminate the now-defunct ifdef for CONFIG_BPF_SYSCALL for
user->locked_vm.

This issue exists between all of the components which use user->locked_vm
and perf, however secretmem is the first user who has caused a report over
many years and has been backported, so target the fix at that.

Fixes: 97d34aa65c29 ("mm/secretmem: properly account locked pages")
Closes: https://lore.kernel.org/20261002175651.811343-1-ameer.hamza@truenas.com
Reported-by: Ameer Hamza <ameer.hamza@truenas.com>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20261003-perf-locked-vm-fix-v1-1-d214dcaf0d76@kernel.org

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