posixfs: investigate preadv2/pwritev2 flag RWF_UNCACHED

There are patches in flight for the linux kernel to add an additional mechanism to allow file IO without affecting file caches. The proposal uses a new flag RWF_UNCACHED for the preadv2 and pwritev2 system calls to activate this mode. Possibly this could be interesting for borg to use instead of POSIX_FADV_DONTNEED.

Points to consider: Does using this for reading make sense even if this disables read-ahead? If missing read ahead is a problem would a reader thread help?

See https://lwn.net/Articles/806980/ for details about the inflight patches.