LoongArch: Set hugetlb mmap base address aligned with pmd size

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

LoongArch: Set hugetlb mmap base address aligned with pmd size

With ltp test case "testcases/bin/hugefork02", there is a dmesg error
report message such as:

kernel BUG at mm/hugetlb.c:5550!
Oops - BUG[#1]:
CPU: 0 UID: 0 PID: 1517 Comm: hugefork02 Not tainted 6.14.0-rc2+ #241
Hardware name: QEMU QEMU Virtual Machine, BIOS unknown 2/2/2022
pc 90000000004eaf1c ra 9000000000485538 tp 900000010edbc000 sp 900000010edbf940
a0 900000010edbfb00 a1 9000000108d20280 a2 00007fffe9474000 a3 00007ffff3474000
a4 0000000000000000 a5 0000000000000003 a6 00000000003cadd3 a7 0000000000000000
t0 0000000001ffffff t1 0000000001474000 t2 900000010ecd7900 t3 00007fffe9474000
t4 00007fffe9474000 t5 0000000000000040 t6 900000010edbfb00 t7 0000000000000001
t8 0000000000000005 u0 90000000004849d0 s9 900000010edbfa00 s0 9000000108d20280
s1 00007fffe9474000 s2 0000000002000000 s3 9000000108d20280 s4 9000000002b38b10
s5 900000010edbfb00 s6 00007ffff3474000 s7 0000000000000406 s8 900000010edbfa08
ra: 9000000000485538 unmap_vmas+0x130/0x218
ERA: 90000000004eaf1c __unmap_hugepage_range+0x6f4/0x7d0
PRMD: 00000004 (PPLV0 +PIE -PWE)
EUEN: 00000007 (+FPE +SXE +ASXE -BTE)
ECFG: 00071c1d (LIE=0,2-4,10-12 VS=7)
ESTAT: 000c0000 [BRK] (IS= ECode=12 EsubCode=0)
PRID: 0014c010 (Loongson-64bit, Loongson-3A5000)
Process hugefork02 (pid: 1517, threadinfo=00000000a670eaf4, task=000000007a95fc64)
Call Trace:
[<90000000004eaf1c>] __unmap_hugepage_range+0x6f4/0x7d0
[<9000000000485534>] unmap_vmas+0x12c/0x218
[<9000000000494068>] exit_mmap+0xe0/0x308
[<900000000025fdc4>] mmput+0x74/0x180
[<900000000026a284>] do_exit+0x294/0x898
[<900000000026aa30>] do_group_exit+0x30/0x98
[<900000000027bed4>] get_signal+0x83c/0x868
[<90000000002457b4>] arch_do_signal_or_restart+0x54/0xfa0
[<90000000015795e8>] irqentry_exit_to_user_mode+0xb8/0x138
[<90000000002572d0>] tlb_do_page_fault_1+0x114/0x1b4

The problem is that base address allocated from hugetlbfs is not aligned
with pmd size. Here add a checking for hugetlbfs and align base address
with pmd size. After this patch the test case "testcases/bin/hugefork02"
passes to run.

This is similar to the commit 7f24cbc9c4d42db8a3c8484d1 ("mm/mmap: teach
generic_get_unmapped_area{_topdown} to handle hugetlb mappings").