btrfs: remove unnecessary btrfs_key local variable in btrfs_search_forward()

The 'found_key' variable was only used to temporarily store the found key
before copying it to 'min_key' at the end of the function when returning
success.

Eliminate the 'found_key' variable, and directly store the key into
'min_key' at the exact loop exit points where ret=0 is set, maintaining
identical functionality.

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