Defer freeing until we drop devmtx ("cdev").
Before r332974 the old code would sometimes cause a rare lock order reversal against pagequeue, which looked roughly like this:
witness_checkorder()
__mtx_lock-flags()
vm_page_alloc()
uma_small_alloc()
keg_alloc_slab()
keg_fetch-slab()
zone_fetch-slab()
zone_import()
zone_alloc_bucket()
uma_zalloc_arg()
bucket_alloc()
uma_zfree_arg()
free()
devfs_metoo()
devfs_populate_loop()
devfs_populate()
devfs_rioctl()
VOP_IOCTL_APV()
VOP_IOCTL()
vn_ioctl()
fo_ioctl()
kern_ioctl()
sys_ioctl()
Since r332974 the original problem no longer exists, but it still makes sense to move things out of the - often congested - lock.