5056 ZFS deadlock on db_mtx and dn_holds
Reviewed by: Will Andrews <willa@spectralogic.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>

*** 118,128 ****
          dsl_dataset_t *bmark_fs;
          char *shortname;
          int error;
          zfs_bookmark_phys_t bmark_phys;
  
!         if (!dsl_dataset_is_snapshot(snapds))
                  return (SET_ERROR(EINVAL));
  
          error = dsl_bookmark_hold_ds(dp, bookmark_name,
              &bmark_fs, FTAG, &shortname);
          if (error != 0)
--- 118,128 ----
          dsl_dataset_t *bmark_fs;
          char *shortname;
          int error;
          zfs_bookmark_phys_t bmark_phys;
  
!         if (!snapds->ds_is_snapshot)
                  return (SET_ERROR(EINVAL));
  
          error = dsl_bookmark_hold_ds(dp, bookmark_name,
              &bmark_fs, FTAG, &shortname);
          if (error != 0)