securityfs: securityfs_remove should handle IS_ERR pointers

Both of the securityfs users (TPM and IMA) can call securityfs_remove and pass
an IS_ERR(dentry) in their failure paths.  This patch handles those rather
than panicing when it tries to start deferencing some negative memory.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>