Race condition between spa async threads and export

In the past we've seen multiple race conditions that have
to do with open-context threads async threads and concurrent
calls to spa_export()/spa_destroy() (including the one
referenced in issue #9015).

This patch ensures that only one thread can execute the
main body of spa_export_common() at a time, with subsequent
threads returning with a new error code created just for
this situation, eliminating this way any race condition
bugs introduced by concurrent calls to this function.

Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #9015 
Closes #9044

include/libzfs.h

@@ -147,6 +147,7 @@ typedef enum zfs_error {
 	EZFS_NO_TRIM,		/* no active trim */
 	EZFS_TRIM_NOTSUP,	/* device does not support trim */
 	EZFS_NO_RESILVER_DEFER,	/* pool doesn't support resilver_defer */
+	EZFS_EXPORT_IN_PROGRESS,	/* currently exporting the pool */
 	EZFS_UNKNOWN
 } zfs_error_t;