mpi3mr: Add atomic decrement-if-not-zero helper

[chandrakanth.patil_broadcom.com]

Summary

Added a generic inline helper function mpi3mr_atomic_dec_if_not_zero() that performs
a safe atomic decrement only if the current value is non-zero.

This wrapper uses atomic_fcmpset_int() to ensure lockless and thread-safe modification of
mpi3mr_atomic_t counters. It replaces non-atomic read-and-decrement patterns in the driver,
which are prone to race conditions and possible underflow under concurrent access.

Suggested by: chs

[chandrakanth.patil_broadcom.com]

Hi Chs,

Thank you for reviewing and suggesting the code.

• This patch fixes the issue for patch https://reviews.freebsd.org/D49749.
• I will submit the other patch soon which would take care of all remaining atomic check and decrement instances in the other code path.

[imp]

I think this is good...

[chandrakanth.patil_broadcom.com]

In D50134#1145957, @imp wrote:

I think this is good...

Thank you for reviewing and approving it!!

[chs]

this is fine as far as it goes, but the other places that already do "mpi3mr_atomic_dec(&tgtdev->block_io)" should also be converted to use the new mpi3mr_atomic_dec_if_not_zero().