nfsd4: don't do lookup within readdir in recovery code

The main nfsd code was recently modified to no longer do lookups from
withing the readdir callback, to avoid locking problems on certain
filesystems.

This (rather hacky, and overdue for replacement) NFSv4 recovery code has
the same problem.  Fix it to build up a list of names (instead of
dentries) and do the lookups afterwards.

Reported symptoms were a deadlock in the xfs code (called from
nfsd4_recdir_load), with /var/lib/nfs on xfs.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Reported-by: David Warren <warren@atmos.washington.edu>