Ensure boot fsread correctly probes all partitions

The boot code fsread was caching the result of meta data request and
reusing it even for calls with inode = 0, which is used to partitions
trigger a probe.

The result was that success was incorrectly returned for all partition
probes after the first valid success, even for partitions which are not
UFS.

MFC after: 2 weeks
X-MFC-With: r293268
Sponsored by: Multiplay