bpf: Implement resizable hashmap basic functions

Use rhashtable_lookup_likely() for lookups, rhashtable_remove_fast()
for deletes, and rhashtable_lookup_get_insert_fast() for inserts.

Updates modify values in place under RCU rather than allocating a
new element and swapping the pointer (as regular htab does). This
trades read consistency for performance: concurrent readers may
see partial updates. BPF_F_LOCK support and special-field
handling (timers, kptrs, etc.) follow in a later commit.

Initialize rhashtable with bpf_mem_alloc element cache. Require
BPF_F_NO_PREALLOC. Limit max_entries to 2^31. Free elements via
rhashtable_free_and_destroy().

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/20260605-rhash-v7-4-5b8e05f8630d@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -1047,6 +1047,7 @@ enum bpf_map_type {
 	BPF_MAP_TYPE_CGRP_STORAGE,
 	BPF_MAP_TYPE_ARENA,
 	BPF_MAP_TYPE_INSN_ARRAY,
+	BPF_MAP_TYPE_RHASH,
 	__MAX_BPF_MAP_TYPE
 };
 
@@ -1545,6 +1546,11 @@ union bpf_attr {
 		 *
 		 * BPF_MAP_TYPE_ARENA - contains the address where user space
 		 * is going to mmap() the arena. It has to be page aligned.
+		 *
+		 * BPF_MAP_TYPE_RHASH - initial table size hint
+		 * (nelem_hint). 0 = use rhashtable default. Must be
+		 * <= min(max_entries, U16_MAX). Upper 32 bits reserved,
+		 * must be zero.
 		 */
 		__u64	map_extra;