x86/speculation: Add missing srbds=off to the mitigations= help text

The mitigations= cmdline option help text misses the srbds=off option.
Add it.

  [ bp: Add a commit message. ]

Signed-off-by: Xiao Yang <yangx.jy@fujitsu.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220513101637.216487-1-yangx.jy@fujitsu.com