NFSv4.2: fix wrong shrinker_id

Currently, the list_lru::shrinker_id corresponding to the nfs4_xattr
shrinkers is wrong:

>>> prog["nfs4_xattr_cache_lru"].shrinker_id
(int)0
>>> prog["nfs4_xattr_entry_lru"].shrinker_id
(int)0
>>> prog["nfs4_xattr_large_entry_lru"].shrinker_id
(int)0
>>> prog["nfs4_xattr_cache_shrinker"].id
(int)18
>>> prog["nfs4_xattr_entry_shrinker"].id
(int)19
>>> prog["nfs4_xattr_large_entry_shrinker"].id
(int)20

This is not what we expect, which will cause these shrinkers
not to be found in shrink_slab_memcg().

We should assign shrinker::id before calling list_lru_init_memcg(),
so that the corresponding list_lru::shrinker_id will be assigned
the correct value like below:

>>> prog["nfs4_xattr_cache_lru"].shrinker_id
(int)16
>>> prog["nfs4_xattr_entry_lru"].shrinker_id
(int)17
>>> prog["nfs4_xattr_large_entry_lru"].shrinker_id
(int)18
>>> prog["nfs4_xattr_cache_shrinker"].id
(int)16
>>> prog["nfs4_xattr_entry_shrinker"].id
(int)17
>>> prog["nfs4_xattr_large_entry_shrinker"].id
(int)18

So just do it.

Fixes: 95ad37f9 ("NFSv4.2: add client side xattr caching.")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>