mount: Clear stbuf in attr_to_stat

Make attr_to_stat overwrite stbuf with zeros before filling it.

This prevents returning garbage values in fields that attr_to_stat
doesn't know how to fill and also silences valgrind 'uninitialized
data' warnings in case when some fields are unused with some kinds
of files.

Change-Id: I62ba4c02b8f36c706de07bed24ab05e388cdd6c8