Revert "Xen/swiotlb: mark xen_swiotlb_fixup() __init"

This reverts commit 1a95cff6 which is
commit 75ad0231 upstream.

Turns out it causes build warnings and might break systems.

Link: https://lore.kernel.org/r/20250407181218.GA737271@ax162


Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Salvatore Bonaccorso <carnil@debian.org>
Cc: Juergen Gross <jgross@suse.com>
Cc: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>