x86/entry/64: Correct the comment over SAVE_AND_SET_GSBASE

Add the proper explanation why an LFENCE is not needed in the FSGSBASE
case.

Fixes: c82965f9 ("x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit")
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200821090710.GE12181@zn.tnic