btrfs: btrfs_issue_discard ensure offset/length are aligned to sector boundaries

It's possible, though unexpected, to pass unaligned offsets and lengths
to btrfs_issue_discard.  We then shift the offset/length values to sector
units.  If an unaligned offset has been passed, it will result in the
entire sector being discarded, possibly losing data.  An unaligned
length is safe but we'll end up returning an inaccurate number of
discarded bytes.

This patch aligns the offset to the 512B boundary, adjusts the length,
and warns, since we shouldn't be discarding on an offset that isn't
aligned with our sector size.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Tested-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>