fs: fix writeback.c 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>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>