ceph: Fix error handling in fill_readdir_cache()

ceph: Fix error handling in fill_readdir_cache()

__filemap_get_folio() returns an ERR_PTR, not NULL.  There are extensive
assumptions that ctl->folio is NULL, not an error pointer, so it seems
better to fix this one place rather than change all the places which
check ctl->folio.

Fixes: baff9740bc8f ("ceph: Convert ceph_readdir_cache_control to store a folio")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20250304154818.250757-1-willy@infradead.org
Cc: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>