bpf: Take mmap_lock in zap_pages()

zap_vma_range() requires the owning mm's mmap_lock to be held.

Taking mmap_read_lock under arena->lock would AB-BA against
arena_vm_close() and arena_map_mmap(), both of which run with
mmap_write_lock held and then acquire arena->lock. Instead drop
arena->lock, mmget_not_zero() the vma's mm, take mmap_read_lock, and
re-resolve the vma via find_vma() since it may have been unmapped or
replaced while waiting.

Track processed vmls with a per-call generation in vml->zap_gen and
serialize zap_pages() callers with a new arena->zap_mutex so
concurrent callers on different uaddr ranges do not mark each other's
vmls processed before the zap is done.

Reported-by: David Hildenbrand <david@kernel.org>
Fixes: 317460317a02 ("bpf: Introduce bpf_arena.")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260528222014.38980-1-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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