Blackfin arch: handle case of d_path() returning error in decode_address()

d_path() can return an error.  Most of its callers do something or other to
make up something sane in that case.  Do similar for blackfin's
decode_address() call to d_path().

Signed-off-by: Tim Pepper <lnxninja@linux.vnet.ibm.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>