reencrypt: use fsync instead of O_DIRECT flag

O_DIRECT operations directed towards filesystem are problematic:
There's no sane way how to detect specific filesystem requirements
for such operations.

This patch is replacing O_DIRECT flag with O_SYNC flag for all
open() calls related to reencrypt log. The O_SYNC flag is used
when --use-fsync option is detected.

Man page is modified accordingly.
reencrypt-compat-test: test log I/Os on various fs
add vfat test image