xattr: add rcu_head and rhash_head to struct simple_xattr

xattr: add rcu_head and rhash_head to struct simple_xattr

In preparation for converting simple_xattrs from rbtree to rhashtable,
add rhash_head and rcu_head members to struct simple_xattr. The
rhashtable implementation will use rhash_head for hash table linkage
and RCU-based lockless reads, requiring that replaced or removed xattr
entries be freed via call_rcu() rather than immediately.

Add simple_xattr_free_rcu() which schedules RCU-deferred freeing of an
xattr entry.  This will be used by callers of simple_xattr_set() once
they switch to the rhashtable-based xattr store.

No functional changes.

Link: https://patch.msgid.link/20260216-work-xattr-socket-v1-1-c2efa4f74cb7@kernel.org
Acked-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>