kernel-doc: track line numbers for each file separately

The problem is that $. keeps track of the current record number (which
is line number by default). But if you pass it multiple files, it does
not wrap at the end of file, and therefore contains the *total* number
of processed lines.
I suppose we can fix line numbering by introducing a simple assignment
$. = 1
before processing every new file.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>