ovl: Fix ovl_getattr() to get number of blocks from lower

ovl: Fix ovl_getattr() to get number of blocks from lower

If an inode has been copied up metadata only, then we need to query the
number of blocks from lower and fill up the stat->st_blocks.

We need to be careful about races where we are doing stat on one cpu and
data copy up is taking place on other cpu.  We want to return
stat->st_blocks either from lower or stable upper and not something in
between.  Hence, ovl_has_upperdata() is called first to figure out whether
block reporting will take place from lower or upper.

We now support metacopy dentries in middle layer.  That means number of
blocks reporting needs to come from lowest data dentry and this could be
different from lower dentry.  Hence we end up making a separate
vfs_getxattr() call for metacopy dentries to get number of blocks.

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