ipv4: fib: Add fib_info_hash_grow().

ipv4: fib: Add fib_info_hash_grow().

When the number of struct fib_info exceeds the hash table size in
fib_create_info(), we try to allocate a new hash table with the
doubled size.

The allocation is done in fib_create_info(), and if successful, each
struct fib_info is moved to the new hash table by fib_info_hash_move().

Let's integrate the allocation and fib_info_hash_move() as
fib_info_hash_grow() to make the following change cleaner.

While at it, fib_info_hash_grow() is placed near other hash-table-specific
functions.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250228042328.96624-8-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>