bpf: Use kmalloc_nolock() in range tree

bpf: Use kmalloc_nolock() in range tree

The range tree uses bpf_mem_alloc() that is safe to be called from all
contexts and uses a pre-allocated pool of memory to serve these
allocations.

Replace bpf_mem_alloc() with kmalloc_nolock() as it can be called safely
from all contexts and is more scalable than bpf_mem_alloc().

Remove the migrate_disable/enable pairs as they were only needed for
bpf_mem_alloc() as it does per-cpu operations, kmalloc_nolock() doesn't
need this.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20251106170608.4800-1-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>