hpfs: fix a crash if hpfs_map_dnode_bitmap fails

If hpfs_map_dnode_bitmap fails, the code would call hpfs_brelse4 on
uninitialized quad buffer head, causing a crash.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reported-by: Farhad Alemi <farhad.alemi@berkeley.edu>
Cc: stable@vger.kernel.org