checkpatch: ensure we actually detect if assignments split across lines

When checking for assignments within if conditionals we check the whole of
the condition, but the match is performed using a line constrained regular
expression.  This means we can miss split conditionals or those on the
second line.  Allow the check to span lines.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>