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Don't mark pages as valid if reading the contents from disk fails.

Description

Don't mark pages as valid if reading the contents from disk fails.
Instead, just skip marking pages valid if the read fails. Future
attempts to access such pages will notice that they are not marked valid
and try to read them from disk again.

Reviewed by: kib, markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D25138