bpf: arena: use kmalloc_nolock() in place of kvcalloc()

bpf: arena: use kmalloc_nolock() in place of kvcalloc()

To make arena_alloc_pages() safe to be called from any context, replace
kvcalloc() with kmalloc_nolock() so as it doesn't sleep or take any
locks. kmalloc_nolock() returns NULL for allocations larger than
KMALLOC_MAX_CACHE_SIZE, which is (PAGE_SIZE * 2) = 8KB on systems with
4KB pages. So, round down the allocation done by kmalloc_nolock to 1024
* 8 and reuse the array in a loop.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20251222195022.431211-3-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>