btrfs: avoid assigning twice to block_start at btrfs_do_readpage()

At btrfs_do_readpage() if we get an extent map for a prealloc extent we
end up assigning twice to the 'block_start' variable, first the value
returned by extent_map_block_start() and then EXTENT_MAP_HOLE. This is
pointless so make it more clear by using an if-else statement and doing
only one assignment. Also, while at it, move the declaration of
'block_start' into the while loop's scope, since it's not used outside of
it and the related 'disk_bytenr' is also declared in this scope.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -939,7 +939,6 @@ static int btrfs_do_readpage(struct folio *folio, struct extent_map **em_cached,
 	u64 cur = start;
 	u64 extent_offset;
 	u64 last_byte = i_size_read(inode);
-	u64 block_start;
 	struct extent_map *em;
 	int ret = 0;
 	size_t pg_offset = 0;
@@ -966,6 +965,7 @@ static int btrfs_do_readpage(struct folio *folio, struct extent_map **em_cached,
 		enum btrfs_compression_type compress_type = BTRFS_COMPRESS_NONE;
 		bool force_bio_submit = false;
 		u64 disk_bytenr;
+		u64 block_start;
 
 		ASSERT(IS_ALIGNED(cur, fs_info->sectorsize));
 		if (cur >= last_byte) {
@@ -991,9 +991,11 @@ static int btrfs_do_readpage(struct folio *folio, struct extent_map **em_cached,
 			disk_bytenr = em->disk_bytenr;
 		else
 			disk_bytenr = extent_map_block_start(em) + extent_offset;
-		block_start = extent_map_block_start(em);
+
 		if (em->flags & EXTENT_FLAG_PREALLOC)
 			block_start = EXTENT_MAP_HOLE;
+		else
+			block_start = extent_map_block_start(em);
 
 		/*
 		 * If we have a file range that points to a compressed extent