NFSD: Cap the number of bytes copied by nfs4_reset_recoverydir()

It's only current caller already length-checks the string, but let's
be safe.

Fixes: 0964a3d3 ("[PATCH] knfsd: nfsd4 reboot dirname fix")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>