luks2: Rename variable "i" to "keyslot_idx" in luks2_get_keyslot()

Variables named "i" are usually looping variables. So, rename it to
"keyslot_idx" to ease luks2_get_keyslot() reading.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>