Drop cache after fsync

Drop the cache (fadvise) after calling fsync to avoid redundant work
in the kernel visiting pages that are dirty. By dropping cache after
fsync(), the pages will be clean so can be easily removed from the
cache.

Change-Id: I3013d2ac1e27bbe43e58a81638d04805f5e2ae6e
Signed-off-by: Peter Portante <peter.portante@redhat.com>