net: sunrpc: Fix 'snprintf' return value check in 'do_xprt_debugfs'

'snprintf' returns the number of characters which would have been written
if enough space had been available, excluding the terminating null byte.
Thus, the return value of 'sizeof(buf)' means that the last character
has been dropped.

Signed-off-by: Fedor Tokarev <ftokarev@gmail.com>
Fixes: 2f34b8bf

 ("SUNRPC: add links for all client xprts to debugfs")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>