btf: Fix virt_to_phys() on arm64 when mmapping BTF

Breno Leitao reports that arm64 emits the following warning
with CONFIG_DEBUG_VIRTUAL:

    [   58.896157] virt_to_phys used for non-linear address: 000000009fea9737
      (__start_BTF+0x0/0x685530)
    [   23.988669] WARNING: CPU: 25 PID: 1442 at arch/arm64/mm/physaddr.c:15
      __virt_to_phys (arch/arm64/mm/physaddr.c:?)

        ...

    [   24.075371] Tainted: [E]=UNSIGNED_MODULE, [N]=TEST
    [   24.080276] Hardware name: Quanta S7GM 20S7GCU0010/S7G MB (CG1), BIOS 3D22
      07/03/2024
    [   24.088295] pstate: 63400009 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
    [   24.098440] pc : __virt_to_phys (arch/arm64/mm/physaddr.c:?)
    [   24.105398] lr : __virt_to_phys (arch/arm64/mm/physaddr.c:?)

	...

    [   24.197257] Call trace:
    [   24.199761] __virt_to_phys (arch/arm64/mm/physaddr.c:?) (P)
    [   24.206883] btf_sysfs_vmlinux_mmap (kernel/bpf/sysfs_btf.c:27)
    [   24.214264] sysfs_kf_bin_mmap (fs/sysfs/file.c:179)
    [   24.218536] kernfs_fop_mmap (fs/kernfs/file.c:462)
    [   24.222461] mmap_region (./include/linux/fs.h:? mm/internal.h:167
       mm/vma.c:2405 mm/vma.c:2467 mm/vma.c:2622 mm/vma.c:2692)

It seems that the memory layout on arm64 maps the kernel image in vmalloc space
which is different than x86. This makes virt_to_phys emit the warning.

Fix this by translating the address using __pa_symbol as suggested by
Breno instead.

Reported-by: Breno Leitao <[email protected]>
Closes: https://lore.kernel.org/bpf/g2gqhkunbu43awrofzqb4cs4sxkxg2i4eud6p4qziwrdh67q4g@mtw3d3aqfgmb/
Signed-off-by: Lorenz Bauer <[email protected]>
Tested-by: Breno Leitao <leitao@debian>
Fixes: a539e2a ("btf: Allow mmap of vmlinux btf")
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: lmb

kernel/bpf/sysfs_btf.c

@@ -21,7 +21,7 @@ static int btf_sysfs_vmlinux_mmap(struct file *filp, struct kobject *kobj,
 {
 	unsigned long pages = PAGE_ALIGN(attr->size) >> PAGE_SHIFT;
 	size_t vm_size = vma->vm_end - vma->vm_start;
-	phys_addr_t addr = virt_to_phys(__start_BTF);
+	phys_addr_t addr = __pa_symbol(__start_BTF);
 	unsigned long pfn = addr >> PAGE_SHIFT;
 
 	if (attr->private != __start_BTF || !PAGE_ALIGNED(addr))