arm64: traps: address fallout from printk -> pr_* conversion

Commit ac7b406c ("arm64: Use pr_* instead of printk") was a fairly
mindless s/printk/pr_*/ change driven by a complaint from checkpatch.

As is usual with such changes, this has led to some odd behaviour on
arm64:

  * syslog now picks up the "pr_emerg" line from dump_backtrace, but not
    the actual trace, which leads to a bunch of "kernel:Call trace:"
    lines in the log

  * __{pte,pmd,pgd}_error print at KERN_CRIT, as opposed to KERN_ERR
    which is used by other architectures.

This patch restores the original printk behaviour for dump_backtrace
and downgrade the pgtable error macros to KERN_ERR.

Signed-off-by: Will Deacon <will.deacon@arm.com>