checkpatch: extend author Signed-off-by check for split From: header

checkpatch: extend author Signed-off-by check for split From: header

Checkpatch did not handle cases where the author From: header was split
into multiple lines.  The author identity could not be resolved and
checkpatch generated a false NO_AUTHOR_SIGN_OFF warning.

A typical example is commit e33bcbab16d1 ("tee: add support for session's
client UUID generation").  When checkpatch was run on this commit, it
displayed:

"WARNING:NO_AUTHOR_SIGN_OFF: Missing Signed-off-by: line by nominal
patch author ''"

This was due to split header lines not being handled properly and the
author himself wrote in commit cd2614967d8b ("checkpatch: warn if missing
author Signed-off-by"):

"Split From: headers are not fully handled: only the first part
is compared."

Support split From: headers by correctly parsing the header extension
lines.  RFC 5322, Section-2.2.3 stated that each extended line must start
with a WSP character (a space or htab).  The solution was therefore to
concatenate the lines which start with a WSP to get the correct long
header.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Joe Perches <joe@perches.com>
Link: https://lore.kernel.org/linux-kernel-mentees/f5d8124e54a50480b0a9fa638787bc29b6e09854.camel@perches.com/
Link: https://lkml.kernel.org/r/20200921085436.63003-1-dwaipayanray1@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>