cachefiles: Fix incorrect length to fallocate()

When cachefiles_shorten_object() calls fallocate() to shape the cache
file to match the DIO size, it passes the total file size it wants to
achieve, not the amount of zeros that should be inserted.  Since this is
meant to preallocate that amount of storage for the file, it can cause
the cache to fill up the disk and hit ENOSPC.

Fix this by passing the length actually required to go from the current
EOF to the desired EOF.

Fixes: 7623ed67 ("cachefiles: Implement cookie resize for truncate")
Reported-by: NJeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Tested-by: NJeff Layton <jlayton@kernel.org>
Reviewed-by: NJeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/164630854858.3665356.17419701804248490708.stgit@warthog.procyon.org.uk # v1
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/cachefiles/interface.c

@@ -254,7 +254,7 @@ static bool cachefiles_shorten_object(struct cachefiles_object *object,
 		ret = cachefiles_inject_write_error();
 		if (ret == 0)
 			ret = vfs_fallocate(file, FALLOC_FL_ZERO_RANGE,
-					    new_size, dio_size);
+					    new_size, dio_size - new_size);
 		if (ret < 0) {
 			trace_cachefiles_io_error(object, file_inode(file), ret,
 						  cachefiles_trace_fallocate_error);