x86/entry/64: Use JMP instead of JMPQ

Somehow the swapgs mitigation entry code patch ended up with a JMPQ
instruction instead of JMP, where only the short jump is needed.  Some
assembler versions apparently fail to optimize JMPQ into a two-byte JMP
when possible, instead always using a 7-byte JMP with relocation.  For
some reason that makes the entry code explode with a #GP during boot.

Change it back to "JMP" as originally intended.

Fixes: 18ec54fd ("x86/speculation: Prepare entry code for Spectre v1 swapgs mitigations")
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

CVE-2019-1125

(backported from commit 64dbc122)
[tyhicks: Adjust context in entry_64.S]
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/x86/entry/entry_64.S

@@ -580,7 +580,7 @@ END(irq_entries_start)
 	call enter_from_user_mode
 #endif
 
-	jmpq	2f
+	jmp	2f
 1:
 	FENCE_SWAPGS_KERNEL_ENTRY
 2: