checkpatch: Fix regexp for if, while, etc inside macros.

checkpatch: Fix regexp for if, while, etc inside macros.

This allows to use a one-character expression inside the 'if'
statement and multiple spaces before the line continuation character.

Fixes false positive in case like this:

  #define MACRO(ARG)     \
      if (a) {           \
          do_work(ARG);  \
      }

Fixes: 16770c6d9179 ("checkpatch: support macro continuation")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Aaron Conole <aconole@redhat.com>