Don't convert all lower-layer errors to EIO.

[imp]

Summary

Don't convert all lower layer errors to EIO. Instead, pass the actual error up
the stack. This will allow the upper layers that look for ENXIO to react
properly to that signal from the lower layers and, for UFS, unmount the
filesystem.

[kib]

sys/vm/vnode_pager.c

631–635

Perhaps assert that error != 0, there and in other chunk.

[imp]

assert error != 0 when B_ERROR flagged