Revert "Add extra EINVAL information about wrong block size to read(2)/write(2)"

This reverts commit 1c2be25f6080ee63baeae55e45761e1310d1b756.

kib@ pointed out that it is perfectly fine to write at arbitrary regular
file offsets. For example, in a 4K block size character device, geom
doesn't support writing / reading 515 byte blocks. The description is
perhaps not applicable to all EINVALs returned.