xfsrestore: untangle inventory unpacking logic

stobj_unpack_sessinfo returns bool_t, fix logic in pi_addfile so errors
can be properly reported.

Signed-off-by: Donald Douwsma <ddouwsma@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>