btrfs-progs: disk-io: Fix read_extent_data() error handler for missing device

btrfs-progs: disk-io: Fix read_extent_data() error handler for missing device

When device is missing, read_extent_data() (function exported from old
btrfs check code) has the following problems:

1) Modifies @len parameter if device is missing
   If device returned in @multi is missing, @len can be larger than
   @max_len (originl length).

   This could confuse caller and underflow in the read loop.

2) Still returns 0 for missing device
   It only handles read error, missing device is not handled and 0 is
   returned.

3) Wrong check for device->fd
   In fact, 0 is also a valid fd.
   Although not possible under most cases, but still needs fix.

Fix them all.

Fixes: 1bad2f2f2dfe ("Btrfs-progs: fsck: add an option to check data csums")
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>