lib/bch.c: use swap() to improve code

lib/bch.c: use swap() to improve code

Use the swap() macro to simplify the functions solve_linear_system() and
gf_poly_gcd() and improve their readability.  Remove the local variable
tmp.

Fixes the following three Coccinelle/coccicheck warnings reported by
swap.cocci:

  WARNING opportunity for swap()
  WARNING opportunity for swap()
  WARNING opportunity for swap()

Link: https://lkml.kernel.org/r/20240708224023.9312-2-thorsten.blum@toblux.com
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>