btrfs: don't warn if discard range is not aligned to sector

There's a warning in btrfs_issue_discard() when the range is not aligned
to 512 bytes, originally added in 4d89d377

 ("btrfs:
btrfs_issue_discard ensure offset/length are aligned to sector
boundaries"). We can't do sub-sector writes anyway so the adjustment is
the only thing that we can do and the warning is unnecessary.

CC: stable@vger.kernel.org # 4.19+
Reported-by:  <syzbot+4a4f1eba14eb5c3417d1@syzkaller.appspotmail.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>