modpost: srcversion sometimes incorrect

"srcversion" field inserted into module modinfo section contains a
sum of the source files which made it. However, this field can
be incorrect. Building the same module can end up having inconsistent
srcversion field eventhough the sources remain the same.
This can be reproduced by building modules in a deeply nested directory,
but other factors contribute as well.

The reason for incorrect srcversion is that some source files can be
simply silently skipped from the checksum calculation due to limited
buffer space for line parsing.

This patch addresses two issues:

1. Allocates a larger line buffer (32k vs 4k).
2. Issues a warning if a line length exceeds the line buffer.

Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>