nfsd: remove /proc/fs/nfs when create /proc/fs/nfs/exports error

when create /proc/fs/nfs/exports error, we should remove /proc/fs/nfs,
if don't do it, it maybe cause Memory leak.

 Signed-off-by: fanchaoting <fanchaoting@cn.fujitsu.com>
 Reviewed-by: chendt.fnst <chendt.fnst@cn.fujitsu.com>

Signed-off-by: J. Bruce Fields <bfields@redhat.com>