swap_pager: Unbusy readahead pages after an I/O error

The swap pager itself allocates readahead pages, so should take care to
unbusy them after a read error, just as it does in the non-error case.

PR: 277538
Reviewed by: olce, dougm, alc, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D44646