Fix various issues with zvols

When performing snapshot renames we could deadlock due to the locking
in zvol_rename_minors. In order to avoid this use the same workaround
as zvol_open in zvol_rename_minors.

Add missing zvol_rename_minors to dsl_dataset_promote_sync.

Protect against invalid index into zv_name in zvol_remove_minors.

Replace zvol_remove_minor calls with zvol_remove_minors to ensure
any potential children are also renamed.

Don't fail zvol_create_minors if zvol_create_minor returns EEXIST.

Restore the valid pool check in zfs_ioc_destroy_snaps to ensure we
don't call zvol_remove_minors when zfs_unmount_snap fails.

PR:		193803
MFC after:	1 week
Sponsored by:	Multiplay


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272474 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f