ovl: set I_CREATING on inode being created

...otherwise there will be list corruption due to inode_sb_list_add() being
called for inode already on the sb list.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: e950564b ("vfs: don't evict uninitialized inode")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>