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

lib/libzpool/kernel.c

@@ -205,6 +205,15 @@ mutex_enter(kmutex_t *mp)
 	mp->m_owner = pthread_self();
 }
 
+int
+mutex_enter_check_return(kmutex_t *mp)
+{
+	int error = pthread_mutex_lock(&mp->m_lock);
+	if (error == 0)
+		mp->m_owner = pthread_self();
+	return (error);
+}
+
 int
 mutex_tryenter(kmutex_t *mp)
 {