btrfs: remove redundant level argument from read_block_for_search()

The level parameter passed to read_block_for_search() always matches the
level of the extent buffer passed in the "eb_ret" parameter, which we are
also extracting into the "parent_level" local variable.

So remove the level parameter and instead use the "parent_level" variable
which in fact has a better name (it's the level of the parent node from
which we are reading a child node/leaf).

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>