F_READAHEAD: Fix r349248's overflow protection, broken by r349391

I accidentally broke the main point of r349248 when making stylistic changes
in r349391. Restore the original behavior, and also fix an additional
overflow that was possible when uio->uio_resid was nearly SSIZE_MAX.

Reported by: cem
Reviewed by: bde
MFC after: 2 weeks
MFC-With: 349248
Sponsored by: The FreeBSD Foundation