SUNRPC: Reinitialise the backchannel request buffers before reuse

stable inclusion
from stable-v4.19.256
commit 1429b2fa358c6fbd5e037edac59e20fa02aa783e
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ


CVE: NA

--------------------------------

commit 6622e3a73112fc336c1c2c582428fb5ef18e456a upstream.

When we're reusing the backchannel requests instead of freeing them,
then we should reinitialise any values of the send/receive xdr_bufs so
that they reflect the available space.

Fixes: 0d2a970d ("SUNRPC: Fix a backchannel race")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>