dmesg: fragment concatenation

dmesg: fragment concatenation

When extended console is enabled, the kernel doesn't internally
concatenate message fragments, this change adds log fragment
concatenation to dmesg: instead of being printed as separate messages,
fragments are combined onto one line and printed with the timestamp of
the first line.

This doesn't work in all cases - such as if another message is logged in
between two fragment lines, but in this case the behavior matches the
previous output format (printed on the separate lines).

[kzak@redhat.com: - rename kmsg_read_buf to kmsg_saved_size,
                  - minor coding style fixes]

Signed-off-by: Vinnie Magro <vmagro@fb.com>
Signed-off-by: Karel Zak <kzak@redhat.com>