bcachefs: bch2_ioctl_subvolume_destroy() fixes

bch2_evict_subvolume_inodes() was getting stuck - due to incorrectly
pruning the dcache.

Also, fix missing permissions checks.

Reported-by: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>