Btrfs: fix that error value is changed by mistake

In iterate_inodes_from_logical() the error result from
extent_from_logical() is patched by mistake. Typically ENOENT is
patched to EINVAL because (-ENOENT & BTRFS_EXTENT_FLAG_TREE_BLOCK)
evaluates to true.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>