RDMA/rxe: Fix the parent sysfs read when the interface has 15 chars

RDMA/rxe: Fix the parent sysfs read when the interface has 15 chars

'parent' sysfs reads will yield '\0' bytes when the interface name has 15
chars, and there will no "\n" output.

To reproduce, create one interface with 15 chars:

 [root@test ~]# ip a s enp0s29u1u7u3c2
 2: enp0s29u1u7u3c2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UNKNOWN group default qlen 1000
     link/ether 02:21:28:57:47:17 brd ff:ff:ff:ff:ff:ff
     inet6 fe80::ac41:338f:5bcd:c222/64 scope link noprefixroute
        valid_lft forever preferred_lft forever
 [root@test ~]# modprobe rdma_rxe
 [root@test ~]# echo enp0s29u1u7u3c2 > /sys/module/rdma_rxe/parameters/add
 [root@test ~]# cat /sys/class/infiniband/rxe0/parent
 enp0s29u1u7u3c2[root@test ~]#
 [root@test ~]# f="/sys/class/infiniband/rxe0/parent"
 [root@test ~]# echo "$(<"$f")"
 -bash: warning: command substitution: ignored null byte in input
 enp0s29u1u7u3c2

Use scnprintf and PAGE_SIZE to fill the sysfs output buffer.

Cc: stable@vger.kernel.org
Fixes: 8700e3e7c485 ("Soft RoCE driver")
Link: https://lore.kernel.org/r/20200820153646.31316-1-yi.zhang@redhat.com
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Yi Zhang <yi.zhang@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>