integrity: fix IMA inode leak

CONFIG_IMA=y inode activity leaks iint_cache and radix_tree_node objects
until the system runs out of memory.  Nowhere is calling ima_inode_free()
a.k.a. ima_iint_delete().  Fix that by calling it from destroy_inode().

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>