s390x/pci: fix DMA slot leak on I/O TLB entry replacement

When s390_pci_update_iotlb() does a mapping update for an IOVA that has
an already-existing TLB entry with different permissions or translated
address, it unmaps then remaps it. However, at the end of the map path,
it unconditionally decrements the available DMA slot counter without a
corresponding increment in the intermediate unmap branch.

This causes the DMA slot count to be decremented on every remapping of
an active IOVA, leading to a permanent DMA slot leak. This remapping
without a prior invalidation and sync is not seen today in well-behaved
guests but is allowed by the architecture. Fix it by only decrementing
available DMA slots when inserting a brand new mapping.

Cc: qemu-stable@nongnu.org
Fixes: 37fa32de70 ("s390x/pci: Honor DMA limits set by vfio")
Signed-off-by: Omar Elghoul <oelghoul@linux.ibm.com>
Reviewed-by: Farhan Ali <alifm@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20260917140431.45343-1-oelghoul@linux.ibm.com
Signed-off-by: Eric Farman <farman@linux.ibm.com>
(cherry picked from commit 9b64b3cc833eaec290e65bc0958aeb51c6892152)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>