mm/mprotect: only reference swap pfn page if type match

In the Linux kernel, the following vulnerability has been resolved:

mm/mprotect: only reference swap pfn page if type match

Yu Zhao reported a bug after the commit "mm/swap: Add swp_offset_pfn() to
fetch PFN from swap entry" added a check in swp_offset_pfn() for swap type [1]:

kernel BUG at include/linux/swapops.h:117!
CPU: 46 PID: 5245 Comm: EventManager_De Tainted: G S O L 6.0.0-dbg-DEV #2
RIP: 0010:pfn_swap_entry_to_page+0x72/0xf0
Code: c6 48 8b 36 48 83 fe ff 74 53 48 01 d1 48 83 c1 08 48 8b 09 f6
c1 01 75 7b 66 90 48 89 c1 48 8b 09 f6 c1 01 74 74 5d c3 eb 9e <0f> 0b
48 ba ff ff ff ff 03 00 00 00 eb ae a9 ff 0f 00 00 75 13 48
RSP: 0018:ffffa59e73fabb80 EFLAGS: 00010282
RAX: 00000000ffffffe8 RBX: 0c00000000000000 RCX: ffffcd5440000000
RDX: 1ffffffffff7a80a RSI: 0000000000000000 RDI: 0c0000000000042b
RBP: ffffa59e73fabb80 R08: ffff9965ca6e8bb8 R09: 0000000000000000
R10: ffffffffa5a2f62d R11: 0000030b372e9fff R12: ffff997b79db5738
R13: 000000000000042b R14: 0c0000000000042b R15: 1ffffffffff7a80a
FS: 00007f549d1bb700(0000) GS:ffff99d3cf680000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000440d035b3180 CR3: 0000002243176004 CR4: 00000000003706e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
change_pte_range+0x36e/0x880
change_p4d_range+0x2e8/0x670
change_protection_range+0x14e/0x2c0
mprotect_fixup+0x1ee/0x330
do_mprotect_pkey+0x34c/0x440
__x64_sys_mprotect+0x1d/0x30

It triggers because pfn_swap_entry_to_page() could be called upon e.g. a
genuine swap entry.

Fix it by only calling it when it's a write migration entry where the page*
is used.

[1] https://lore.kernel.org/lkml/CAOUHufaVC2Za-p8m0aiHw6YkheDcrO-C3wRGixwDS32VTS+k1w@mail.gmail.com/