kernel-doc: skip nested struct/union cleanly

Fix handling of nested structs or unions.  The regex to strip (eliminate)
nested structs or unions was limited to only 0 or 1 matches.  This can
cause an uneven number of left/right braces to be stripped, which causes
this:

Warning(linux-2.6.27-rc1-git2//include/net/mac80211.h:336): No description found for parameter '}'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>