percpu-rwlock: introduce support for blocking speculation into critical regions

percpu-rwlock: introduce support for blocking speculation into critical regions

Add direct calls to block_lock_speculation() where required in order to prevent
speculation into the lock protected critical regions.  Also convert
_percpu_read_lock() from inline to always_inline.

Note that _percpu_write_lock() has been modified the use the non speculation
safe of the locking primites, as a speculation is added unconditionally by the
calling wrapper.

This is part of XSA-453 / CVE-2024-2193

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>