dd iflag=nocache should silently ignore fadvise's error in some cases

[oech3]

> strace -qqq -e trace='/fadvise' gnu9.11/dd if=/etc/pacman.conf iflag=nocache>/dev/null
fadvise64(0, 0, 0, POSIX_FADV_DONTNEED) = 0
1+1 records in
1+1 records out
535 bytes copied, 0.000246046 s, 2.2 MB/s

> strace -o /dev/null -e inject=fadvise64:error=ENOSYS gnu9.11/dd if=/etc/pacman.conf iflag=nocache >/dev/null
1+1 records in
1+1 records out
535 bytes copied, 0.000378318 s, 1.4 MB/s

> strace -o /dev/null -e inject=fadvise64:error=ENOSYS dd if=/etc/pacman.conf iflag=nocache >/dev/null
dd: failed to discard cache for: /etc/pacman.conf: Function not implemented
dd: failed to discard cache for: /etc/pacman.conf: Function not implemented
dd: failed to discard cache for: /etc/pacman.conf: Function not implemented
1+1 records in
1+1 records out
535 bytes copied, 0.00134043 s, 535 kB/s
# exit 1

[oech3]

This is not too easy. GNU catches error in some cases too...