fs: btrfs: Fix tree traversal with btrfs_next_slot()

fs: btrfs: Fix tree traversal with btrfs_next_slot()

When traversing slots in a btree (via btrfs_path) with btrfs_next_slot(),
we didn't correctly identify that the last slot in the leaf was reached
and we should jump to the next leaf.

This could lead to any kind of runtime errors or corruptions, like:
* file data not being read at all, or is read partially
* file is read but is corrupted
* (any) metadata being corrupted or not read at all, etc

The easiest way to reproduce this is to read a large enough file that
its EXTENT_DATA items don't fit into a single leaf.

Signed-off-by: Yevgeny Popovych <yevgenyp@pointgrab.com>
Cc: Marek Behun <marek.behun@nic.cz>
Tested-by: Marek Behún <marek.behun@nic.cz>