ceph: Mark the file cache as unreclaimable

Ceph creates multiple caches with the SLAB_RECLAIMABLE flag set, so
that it can satisfy its internal needs. Inspecting the code shows that
most of the caches are indeed reclaimable since they are directly
related to the generic inode/dentry shrinkers. However, one of the
cache used to satisfy struct file is not reclaimable since its
entries are freed only when the last reference to the file is
dropped. If a heavily loaded node opens a lot of files it can
introduce non-trivial discrepancies between memory shown as reclaimable
and what is actually reclaimed when drop_caches is used.

Fix this by removing the reclaimable flag for the file's cache.

Signed-off-by: Nikolay Borisov <n.borisov.lkml@gmail.com>
Signed-off-by: Yan, Zheng <zyan@redhat.com>