checkpatch: add new warnings to author signoff checks.

checkpatch: add new warnings to author signoff checks.

The author signed-off-by checks are currently very vague.  Cases like same
name or same address are not handled separately.

For example, running checkpatch on commit be6577af0cef ("parisc: Add
atomic64_set_release() define to avoid CPU soft lockups"), gives:

WARNING: Missing Signed-off-by: line by nominal patch author
'John David Anglin <[email protected]>'

The signoff line was:
"Signed-off-by: Dave Anglin <[email protected]>"

Clearly the author has signed off but with a slightly different version
of his name. A more appropriate warning would have been to point out
at the name mismatch instead.

Previously, the values assumed by $authorsignoff were either 0 or 1
to indicate whether a proper sign off by author is present.
Extended the checks to handle four new cases.

$authorsignoff values now denote the following:

0: Missing sign off by patch author.

1: Sign off present and identical.

2: Addresses and names match, but comments differ.
   "James Watson(JW) <[email protected]>", "James Watson <[email protected]>"

3: Addresses match, but names are different.
   "James Watson <[email protected]>", "James <[email protected]>"

4: Names match, but addresses are different.
   "James Watson <[email protected]>", "James Watson <[email protected]>"

5: Names match, addresses excluding subaddress details (RFC 5233) match.
   "James Watson <[email protected]>", "James Watson <[email protected]>"

Also introduced a new message type FROM_SIGN_OFF_MISMATCH
for cases 2, 3, 4 and 5.

Suggested-by: Joe Perches <[email protected]>
Signed-off-by: Dwaipayan Ray <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Joe Perches <[email protected]>
Link: https://lore.kernel.org/linux-kernel-mentees/[email protected]/
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>