ARM: fix hash_name() fault

Zizhi Wo reports:

"During the execution of hash_name()->load_unaligned_zeropad(), a
potential memory access beyond the PAGE boundary may occur. For
example, when the filename length is near the PAGE_SIZE boundary.
This triggers a page fault, which leads to a call to
do_page_fault()->mmap_read_trylock(). If we can't acquire the lock,
we have to fall back to the mmap_read_lock() path, which calls
might_sleep(). This breaks RCU semantics because path lookup occurs
under an RCU read-side critical section."

This is seen with CONFIG_DEBUG_ATOMIC_SLEEP=y and CONFIG_KFENCE=y.

Kernel addresses (with the exception of the vectors/kuser helper
page) do not have VMAs associated with them. If the vectors/kuser
helper page faults, then there are two possibilities:

1. if the fault happened while in kernel mode, then we're basically
dead, because the CPU won't be able to vector through this page
to handle the fault.
2. if the fault happened while in user mode, that means the page was
protected from user access, and we want to fault anyway.

Thus, we can handle kernel addresses from any context entirely
separately without going anywhere near the mmap lock. This gives us
an entirely non-sleeping path for all kernel mode kernel address
faults.

As we handle the kernel address faults before interrupts are enabled,
this change has the side effect of improving the branch predictor
hardening, but does not completely solve the issue.

Reported-by: Zizhi Wo <wozizhi@huaweicloud.com>
Reported-by: Xie Yuanbin <xieyuanbin1@huawei.com>
Link: https://lore.kernel.org/r/20251126090505.3057219-1-wozizhi@huaweicloud.com
Reviewed-by: Xie Yuanbin <xieyuanbin1@huawei.com>
Tested-by: Xie Yuanbin <xieyuanbin1@huawei.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>