mm/readahead: Fix null pointer

mm/readahead: Fix null pointer

When using BTRFS, the kernel panics on boot during mount with the following
error message: `Unable to handle kernel NULL pointer dereference at virtual
address 00000044`

The responsible code snippet has been added to mm/readahead and contains a
null pointer: `filp` can be NULL when using filesystems writing metadata
(e.g. btrfs)

Fix this issue by extending the if-clause