btrfs: fix lost return value due to variable shadowing

commit 1932b7be973b554ffe20a5bba6ffaed6fa995cdc upstream.

A block-local variable stores error code but btrfs_get_blocks_direct may
not return it in the end as there's a ret defined in the function scope.

Fixes: d187663e ("Btrfs: lock extents as we map them in DIO")
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>