fs/writeback.c: fix kernel-doc warnings

Fix kernel-doc warnings in fs/fs-writeback.c by moving a #define macro to
after the function's opening brace.  Also #undef this macro at the end of
the function.

  ../fs/fs-writeback.c:1984: warning: Excess function parameter 'inode' description in 'I_DIRTY_INODE'
  ../fs/fs-writeback.c:1984: warning: Excess function parameter 'flags' description in 'I_DIRTY_INODE'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>