fs: drop redundant check from __writeback_single_inode()

wbc->for_sync implies wbc->sync_mode == WB_SYNC_ALL, so there's no need
to check for both.  Just check for WB_SYNC_ALL.

Link: https://lore.kernel.org/r/20210112190253.64307-9-ebiggers@kernel.org


Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jan Kara <jack@suse.cz>