nfs: get rid of fake root dentries

nfs: get rid of fake root dentries

... just grab the reference to the (real) root we are about to return
for the first mount of this superblock and be done with that.

Once upon a time dentry tree eviction at fs shutdown used to break
if ->s_root had been spliced on top of something; that hadn't been
the case for years now, and these fake root dentries violate a bunch
of invariants.  Let's get rid of them...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>