octeontx2-af: npc: cn20k: Propagate errors in defrag MCAM alloc rollback

npc_defrag_alloc_free_slots() allocates MCAM indexes in up to two passes on
bank0 then bank1. On failure it rolls back by freeing entries already
placed in save[].

__npc_subbank_alloc() can return a negative errno while only part of the
indexes are valid. The rollback loop used rc for
npc_mcam_idx_2_subbank_idx() as well, so a successful lookup stored zero in
rc and a later __npc_subbank_free() failure could still end with return 0
when the allocation path had also left rc at zero (for example shortfall
after zero return values from the alloc helpers).

Jump to the rollback path immediately when either __npc_subbank_alloc()
call fails, preserving its errno. If both calls succeed but the total
allocated count is still less than cnt, set rc to -ENOSPC before rollback.
Use a separate err variable for npc_mcam_idx_2_subbank_idx() so a
successful lookup no longer clears a non-zero rc from the allocation phase.

Cc: Dan Carpenter <error27@gmail.com>
Fixes: 645c6e3c1999 ("octeontx2-af: npc: cn20k: virtual index support")
Link: https://lore.kernel.org/netdev/adjNJEpILRZATB2N@stanley.mountain/
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Ratheesh Kannoth <rkannoth@marvell.com>
Link: https://patch.msgid.link/20260429022722.1110289-4-rkannoth@marvell.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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