mm/page-writeback.c: write_cache_pages(): deduplicate identical checks

There used to be a 'retry' label in between the two (identical) checks
when first introduced in commit f446daae ("mm: implement writeback
livelock avoidance using page tagging"), and later modified/updated in
commit 6e6938b6 ("writeback: introduce .tagged_writepages for the
WB_SYNC_NONE sync stage").

The label has been removed in commit 64081362

 ("mm/page-writeback.c:
fix range_cyclic writeback vs writepages deadlock"), and the (identical)
checks are now present / performed immediately one after another.

So, remove/deduplicate the latter check, moving tag_pages_for_writeback()
into the former check before the 'tag' variable assignment, so it's clear
that it's not used in this (similarly-named) function call but only later
in pagevec_lookup_range_tag().

Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jan Kara <jack@suse.cz>
Link: http://lkml.kernel.org/r/20200218221716.1648-1-mfo@canonical.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>