ovl: respect FIEMAP_FLAG_SYNC flag

Stacked overlayfs fiemap operation broke xfstests that test delayed
allocation (with "_test_generic_punch -d"), because ovl_fiemap()
failed to write dirty pages when requested.

Fixes: 9e142c41 ("ovl: add ovl_fiemap()")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>