dm verity: Fix compilation warning

mainline inclusion
from mainline-v5.9-rc1
commit a84c4308
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5CES3


CVE: NA

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

For the case !CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG, declare the
functions verity_verify_root_hash(), verity_verify_is_sig_opt_arg(),
verity_verify_sig_parse_opt_args() and verity_verify_sig_opts_cleanup()
as inline to avoid a "no previous prototype for xxx" compilation
warning when compiling with W=1.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Luo Meng <luomeng12@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>