NTB/msi: Switch MSI descriptor locking to lock guard()

Convert the code to use the new guard(msi_descs_lock).

No functional change intended.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/all/20250319105506.263456735@linutronix.de