Add mutex_enter_interruptible() for interruptible sleeping IOCTLs

Many long-running ZFS ioctls lock the spa_namespace_lock, forcing
concurrent ioctls to sleep for the mutex. Previously, the only
option is to call mutex_enter() which sleeps uninterruptibly. This
is a usability issue for sysadmins, for example, if the admin runs
`zpool status` while a slow `zpool import` is ongoing, the admin's
shell will be locked in uninterruptible sleep for a long time.

This patch resolves this admin usability issue by introducing
mutex_enter_interruptible() which sleeps interruptibly while waiting
to acquire a lock. It is implemented for both Linux and FreeBSD.

The ZFS_IOC_POOL_CONFIGS ioctl, used by `zpool status`, is changed to
use this new macro so that the command can be interrupted if it is
issued during a concurrent `zpool import` (or other long-running
operation).

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Thomas Bertschinger <bertschinger@lanl.gov>
Closes #15360

include/os/freebsd/spl/sys/mutex.h

@@ -64,6 +64,7 @@ typedef enum {
 } while (0)
 #define	mutex_destroy(lock)	sx_destroy(lock)
 #define	mutex_enter(lock)	sx_xlock(lock)
+#define	mutex_enter_interruptible(lock)	sx_xlock_sig(lock)
 #define	mutex_enter_nested(lock, type)	sx_xlock(lock)
 #define	mutex_tryenter(lock)	sx_try_xlock(lock)
 #define	mutex_exit(lock)	sx_xunlock(lock)