fold do_sync_file_range into sys_sync_file_range

We recently go rid of all callers of do_sync_file_range as they're better
served with vfs_fsync or the filemap_write_and_wait.  Now that
do_sync_file_range is down to a single caller fold it into it so that people
don't start using it again accidentally.  While at it also switch it from
using __filemap_fdatawrite_range(..., WB_SYNC_ALL) to the more clear
filemap_fdatawrite_range().

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>