btrfs: avoid redundant path slot assignment in btrfs_search_forward()

Move path slot assignment before the condition check to prevent
duplicate assignment. Previously, the slot was set both inside and after
the 'slot >= nritems' block with no change in its value, which is
unnecessary.

Signed-off-by: Sun YangKai <sunk67188@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>