cachefiles: Check that the backing filesystem supports tmpfiles

Add a check that the backing filesystem supports the creation of
tmpfiles[1].
Suggested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/568749bd7cc02908ecf6f3d6a611b6f9cf5c4afd.camel@kernel.org/ [1]
Link: https://lore.kernel.org/r/164251406558.3435901.1249023136670058162.stgit@warthog.procyon.org.uk/ # v1

fs/cachefiles/cache.c

@@ -51,6 +51,7 @@ int cachefiles_add_cache(struct cachefiles_cache *cache)
 
 	/* Check features of the backing filesystem:
 	 * - Directories must support looking up and directory creation
+	 * - We create tmpfiles to handle invalidation
 	 * - We use xattrs to store metadata
 	 * - We need to be able to query the amount of space available
 	 * - We want to be able to sync the filesystem when stopping the cache
@@ -60,6 +61,7 @@ int cachefiles_add_cache(struct cachefiles_cache *cache)
 	if (d_is_negative(root) ||
 	    !d_backing_inode(root)->i_op->lookup ||
 	    !d_backing_inode(root)->i_op->mkdir ||
+	    !d_backing_inode(root)->i_op->tmpfile ||
 	    !(d_backing_inode(root)->i_opflags & IOP_XATTR) ||
 	    !root->d_sb->s_op->statfs ||
 	    !root->d_sb->s_op->sync_fs ||