ima: only insert at inode creation time

iints are supposed to be allocated when an inode is allocated (during
security_inode_alloc())  But we have code which will attempt to allocate
an iint during measurement calls.  If we couldn't allocate the iint and we
cared, we should have died during security_inode_alloc().  Not make the
code more complex and less efficient.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>