fs: protect backing files with rcu

Currently backing files are not under any form of rcu protection.
Switching to file_ref requires rcu protection and so does the
speculative vma lookup. Switch backing files to the same rcu slab type
as regular files. There should be no additional magic required as the
lifetime of a backing file is always tied to a regular file.

Link: https://lore.kernel.org/r/20241007-brauner-file-rcuref-v2-1-387e24dc9163@kernel.org


Signed-off-by: Christian Brauner <brauner@kernel.org>