printk: guard the amount written per line by devkmsg_read()

printk: guard the amount written per line by devkmsg_read()

devkmsg_read() uses 8k buffer and assumes that the formatted output
message won't overrun which seems safe given LOG_LINE_MAX, the current
use of dict and the escaping method being used; however, we're
planning to use devkmsg formatting wider and accounting for the buffer
size properly isn't that complicated.

This patch defines CONSOLE_EXT_LOG_MAX as 8192 and updates
devkmsg_read() so that it limits output accordingly.

Change-Id: Ic8579ddcd55294a38561e9e8b28449c067600db1
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Petr Mladek <pmladek@suse.cz>