kernel-doc: bugfix - multi-line macros

Prior to this patch the following code breaks:

/**
 * multiline_example - this breaks kernel-doc
 */
 #define multiline_example( \
myparam)

Producing this error:

Error(somefile.h:983): cannot understand prototype: 'multiline_example( \ '

This patch fixes the issue by appending all lines ending in a blackslash
(optionally followed by whitespace), removing the backslash and any
whitespace after it prior to appending (just like the C pre-processor
would).

This fixes a break in kerel-doc introduced by the additions to rbtree.h.

Signed-off-by: Daniel Santos <daniel.santos@pobox.com>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>