powerpc/xmon: make dumping log buffer contents more reliable

powerpc/xmon: make dumping log buffer contents more reliable

Log buffer entries that are too long for dump_log_buf()'s small
local buffer are:

* silently discarded when a single-line entry is too long;
  kmsg_dump_get_line() returns true but sets &len to 0.
* silently truncated to the last fitting new line when a multi-line
  entry is too long, e.g. register dumps from __show_regs(); this
  seems undetectable via the kmsg_dump API.

xmon_printf()'s internal buffer is already 1KB; enlarge
dump_log_buf()'s own buffer to match and make it statically
allocated. Verified that this allows complete printing of register
dumps on ppc64le with both CONFIG_PRINTK_TIME=y and
CONFIG_PRINTK_CALLER=y.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210514162420.2911458-1-nathanl@linux.ibm.com