makefs: do not call brelse if bread returns an error

If bread returns an error there is no bp to brelse. One of these
changes was taken from NetBSD commit 0a62dad69f62 ("This works well
enough to populate..."), the rest were found by looking for the same
pattern.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39069

(cherry picked from commit bc33c99234a676c41cde63cce389dd6b8b70adfc)