9p: Use hashtable.h for hash_errmap

Convert hash_errmap in error.c to use the generic hashtable
implementation from hashtable.h instead of the manual hlist_head array
implementation.

This simplifies the code and makes it more maintainable by using the
standard hashtable API and removes the need for manual hash table
management.

Signed-off-by: Sasha Levin <sashal@kernel.org>
Message-ID: <20250320145200.3124863-1-sashal@kernel.org>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>