SUNRPC: Fix READ_PLUS crasher

stable inclusion
from stable-4.19.251
commit 917d77f59a77cab142a6758984d506addb9d414b
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5I4FP


CVE: NA

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

commit a23dd544 upstream.

Looks like there are still cases when "space_left - frag1bytes" can
legitimately exceed PAGE_SIZE. Ensure that xdr->end always remains
within the current encode buffer.

Reported-by: Bruce Fields <bfields@fieldses.org>
Reported-by: Zorro Lang <zlang@redhat.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216151


Fixes: 6c254bf3 ("SUNRPC: Fix the calculation of xdr->end in xdr_get_next_encode_buffer()")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>