nfsd: Fix possible BUG_ON firing in set_change_info

nfsd: Fix possible BUG_ON firing in set_change_info

If vfs_getattr in fill_post_wcc returns an error, we don't
set fh_post_change.
For NFSv4, this can result in set_change_info triggering a BUG_ON.
i.e. fh_post_saved being zero isn't really a bug.

So:
 - instead of BUGging when fh_post_saved is zero, just clear ->atomic.
 - if vfs_getattr fails in fill_post_wcc, take a copy of i_ctime anyway.
   This will be used i seg_change_info, but not overly trusted.
 - While we are there, remove the pointless 'if' statements in set_change_info.
   There is no harm setting all the values.

Signed-off-by: NeilBrown <neilb@suse.de>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>