cachefiles: Implement a function to get/create a directory in the cache

cachefiles: Implement a function to get/create a directory in the cache

Implement a function to get/create structural directories in the cache.
This is used for setting up a cache and creating volume substructures.  The
directory in memory are marked with the S_KERNEL_FILE inode flag whilst
they're in use to tell rmdir to reject attempts to remove them.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com