dmesg: fix off-by-one read buffer size

dmesg: fix off-by-one read buffer size

PRINTK_MESSAGE_MAX is 2048 in the kernel. In a formatted record that is
exactly 2048 bytes, reading /proc/kmsg with a size of PRINTK_MESSAGE_MAX
- 1 (2047) will result in the read syscall returning -EINVAL.

We see such a case when using a large initrd, for which the kernel
outputs loading spinner characters, based on the size of the initrd. For
a large enough initrd, there will be enough spinner characters to create
several formatted records of size 2048.

We fix this by increasing the kmsg_buf size by 1, which increases the
size used by the read syscall to PRINTK_MESSAGE_MAX (2048).

Signed-off-by: Brian Mak <makb@juniper.net>