nfsd: potential ERR_PTR dereference on exp_export() error paths.

We "goto finish" from several places where "exp" is an ERR_PTR.  Also I
changed the check for "fsid_key" so that it was consistent with the check
I added.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>