ovl: Open file with data except for the case of fsync

ovl_open() should open file which contains data and not open metacopy
inode.  With the introduction of metacopy inodes, with current
implementaion we will end up opening metacopy inode as well.

But there can be certain circumstances like ovl_fsync() where we want to
allow opening a metacopy inode instead.

Hence, change ovl_open_realfile() and and add extra parameter which
specifies whether to allow opening metacopy inode or not.  If this
parameter is false, we look for data inode and open that.

This should allow covering both the cases.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>