checkpatch: make email address check case insensitive

Instead of checkpatch requiring the patch author to exactly match the
signed-off-by tag, commit 48ca2d8a

 ("checkpatch: add new warnings to
author signoff checks.") safely relaxed this requirement.

Although the local-part of an email address (local-part@domain), may be
case sensitive, exploiting the case sensitivity of mailbox local-parts
impedes interoperability and is discouraged.  Mailbox domains follow
normal DNS rules and are hence not case sensitive.  (Refer to
https://datatracker.ietf.org/doc/html/rfc5321#section-2.4.)

Further relax the patch author and signed-off-by tag comparison by making
the email address check case insensitive.

Link: https://lkml.kernel.org/r/20210816112725.173206-1-zohar@linux.ibm.com
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>