x86/pti: Make sure the user/kernel PTEs match

Meelis reported that his K8 Athlon64 emits MCE warnings when PTI is
enabled:

[Hardware Error]: Error Addr: 0x0000ffff81e000e0
[Hardware Error]: MC1 Error: L1 TLB multimatch.
[Hardware Error]: cache level: L1, tx: INSN

The address is in the entry area, which is mapped into kernel _AND_ user
space. That's special because we switch CR3 while we are executing
there. 

User mapping:
0xffffffff81e00000-0xffffffff82000000           2M     ro         PSE     GLB x  pmd

Kernel mapping:
0xffffffff81000000-0xffffffff82000000          16M     ro         PSE         x  pmd

So the K8 is complaining that the TLB entries differ. They differ in the
GLB bit.

Drop the GLB bit when installing the user shared mapping.

Fixes: 6dc72c3c ("x86/mm/pti: Share entry text PMD")
Reported-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Meelis Roos <mroos@linux.ee>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/alpine.DEB.2.20.1801031407180.1957@nanos