We've got two issues with the non-regular file handling for non-blocking
IO:
1) We don't want to re-do a short read in full for a non-regular file,
as we can't just read the data again.
2) For non-regular files that don't support non-blocking IO attempts,
we need to punt to async context even if the file is opened as
non-blocking. Otherwise the caller always gets -EAGAIN.
Add two new request flags to handle these cases. One is just a cache
of the inode S_ISREG() status, the other tells io_uring that we always
need to punt this request to async context, even if REQ_F_NOWAIT is set.