fs: lockd: avoid possible wrong NULL parameter

stable inclusion
from stable-v4.19.295
commit 35f0749756b848ad4f4a165ad6b1dfa8d0e45a96
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8E5Q5
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=35f0749756b848ad4f4a165ad6b1dfa8d0e45a96



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[ Upstream commit de8d38cf ]

clang's static analysis warning: fs/lockd/mon.c: line 293, column 2:
Null pointer passed as 2nd argument to memory copy function.

Assuming 'hostname' is NULL and calling 'nsm_create_handle()', this will
pass NULL as 2nd argument to memory copy function 'memcpy()'. So return
NULL if 'hostname' is invalid.

Fixes: 77a3ef33 ("NSM: More clean up of nsm_get_handle()")
Signed-off-by: Su Hui <suhui@nfschina.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: ZhaoLong Wang <wangzhaolong1@huawei.com>