Merge patch series "fs,mm: add kmem_cache_create_rcu()"

Christian Brauner <brauner@kernel.org> says:

When a kmem cache is created with SLAB_TYPESAFE_BY_RCU the free pointer
must be located outside of the object because we don't know what part of
the memory can safely be overwritten as it may be needed to prevent
object recycling.

That has the consequence that SLAB_TYPESAFE_BY_RCU may end up adding a
new cacheline. This is the case for e.g., struct file. After having it
shrunk down by 40 bytes and having it fit in three cachelines we still
have SLAB_TYPESAFE_BY_RCU adding a fourth cacheline because it needs to
accommodate the free pointer.

Add a new kmem_cache_create_rcu() function that allows the caller to
specify an offset where the free pointer is supposed to be placed.

Before this series cat /proc/slabinfo:

filp                1198   1248    256   32    2 : tunables    0    0    0 : slabdata     39     39      0
                                   ^^^

After this series cat /proc/slabinfo:

filp                1323   1323    192   21    1 : tunables    0    0    0 : slabdata     63     63      0
                                   ^^^
* patches from https://lore.kernel.org/r/20240828-work-kmem_cache-rcu-v3-0-5460bc1f09f6@kernel.org:
  fs: use kmem_cache_create_rcu()
  mm: add kmem_cache_create_rcu()
  mm: remove unused root_cache argument

Link: https://lore.kernel.org/r/20240828-work-kmem_cache-rcu-v3-0-5460bc1f09f6@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>