mm: fix lockdep issues in writeback handling

Commit 2841808f ("mm: remove BDI_CAP_WRITEBACK_ACCT") removed
BDI_CAP_WRITEBACK_ACCT flag and refactored code that depend on it.
Unfortunately it also moved some variable intialization out of guarded
scope in writeback handling, what triggers a true lockdep warning. Fix
this by moving initialization to the proper place.

Fixes: 2841808f ("mm: remove BDI_CAP_WRITEBACK_ACCT")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>