btrfs: remove unused BTRFS_MAP_DISCARD

BTRFS_MAP_DISCARD is never set, as REQ_OP_DISCARD is never passed to
btrfs_op() only only checked in two ASSERTS.

Remove it and let the catchall WARN_ON in btrfs_op() deal with accidental
REQ_OP_DISCARDs leaked into btrfs_op(). Last use was in a4012f06
("btrfs: split discard handling out of btrfs_map_block").
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -6182,8 +6182,6 @@ static u64 btrfs_max_io_len(struct map_lookup *map, enum btrfs_map_op op,
 			    u64 offset, u32 *stripe_nr, u64 *stripe_offset,
 			    u64 *full_stripe_start)
 {
-	ASSERT(op != BTRFS_MAP_DISCARD);
-
 	/*
 	 * Stripe_nr is the stripe where this block falls.  stripe_offset is
 	 * the offset of this block in its stripe.
@@ -6261,7 +6259,6 @@ int __btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
 	u64 max_len;
 
 	ASSERT(bioc_ret);
-	ASSERT(op != BTRFS_MAP_DISCARD);
 
 	num_copies = btrfs_num_copies(fs_info, logical, fs_info->sectorsize);
 	if (mirror_num > num_copies)

fs/btrfs/volumes.h

@@ -556,15 +556,12 @@ struct btrfs_dev_lookup_args {
 enum btrfs_map_op {
 	BTRFS_MAP_READ,
 	BTRFS_MAP_WRITE,
-	BTRFS_MAP_DISCARD,
 	BTRFS_MAP_GET_READ_MIRRORS,
 };
 
 static inline enum btrfs_map_op btrfs_op(struct bio *bio)
 {
 	switch (bio_op(bio)) {
-	case REQ_OP_DISCARD:
-		return BTRFS_MAP_DISCARD;
 	case REQ_OP_WRITE:
 	case REQ_OP_ZONE_APPEND:
 		return BTRFS_MAP_WRITE;