x86/vsyscall: Reorganize the page fault emulation code

x86/vsyscall: Reorganize the page fault emulation code

With LASS, vsyscall page accesses will cause a #GP instead of a #PF.
Separate out the core vsyscall emulation code from the #PF specific
handling in preparation for the upcoming #GP emulation.

No functional change intended.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Link: https://patch.msgid.link/20260309181029.398498-2-sohil.mehta@intel.com