PCI/MSI: Switch msix_capability_init() to guard(msi_desc_lock)

Split the lock protected functionality of msix_capability_init() out into a
helper function and use guard(msi_desc_lock) to replace the lock/unlock
pair.

Simplify the error path in the helper function by utilizing a custom
cleanup to get rid of the remaining gotos.

No functional change intended.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/all/20250319105506.564105011@linutronix.de