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: c82965f ("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

Author: suryasaimadhu

arch/x86/entry/entry_64.S

@@ -840,8 +840,9 @@ SYM_CODE_START_LOCAL(paranoid_entry)
 	 * retrieve and set the current CPUs kernel GSBASE. The stored value
 	 * has to be restored in paranoid_exit unconditionally.
 	 *
-	 * The MSR write ensures that no subsequent load is based on a
-	 * mispredicted GSBASE. No extra FENCE required.
+	 * The unconditional write to GS base below ensures that no subsequent
+	 * loads based on a mispredicted GS base can happen, therefore no LFENCE
+	 * is needed here.
 	 */
 	SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx
 	ret