exec: binfmt_misc: don't nullify Node->dentry in kill_node()

exec: binfmt_misc: don't nullify Node->dentry in kill_node()

kill_node() nullifies/checks Node->dentry to avoid double free.  This
complicates the next changes and this is very confusing:

 - we do not need to check dentry != NULL under entries_lock,
   kill_node() is always called under inode_lock(d_inode(root)) and we
   rely on this inode_lock() anyway, without this lock the
   MISC_FMT_OPEN_FILE cleanup could race with itself.

 - if kill_inode() was already called and ->dentry == NULL we should not
   even try to close e->interp_file.

We can change bm_entry_write() to simply check !list_empty(list) before
kill_node.  Again, we rely on inode_lock(), in particular it saves us
from the race with bm_status_write(), another caller of kill_node().

Link: http://lkml.kernel.org/r/20170922143641.GA17210@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ben Woodard <woodard@redhat.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Jim Foraker <foraker1@llnl.gov>
Cc: <tdhooge@llnl.gov>
Cc: Travis Gummels <tgummels@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>