Bluetooth: increase BTNAMSIZ to 21 chars to fix potential buffer overflow

An earlier commit replaced using batostr to using %pMR sprintf for the
construction of session->name. Static analysis detected that this new
method can use a total of 21 characters (including the trailing '\0')
so we need to increase the BTNAMSIZ from 18 to 21 to fix potential
buffer overflows.

Addresses-Coverity: ("Out-of-bounds write")
Fixes: fcb73338 ("Bluetooth: Use %pMR in sprintf/seq_printf instead of batostr")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>