SELinux: better printk when file with invalid label found

Currently when an inode is read into the kernel with an invalid label
string (can often happen with removable media) we output a string like:

SELinux: inode_doinit_with_dentry:  context_to_sid([SOME INVALID LABEL])
returned -22 dor dev=[blah] ino=[blah]

Which is all but incomprehensible to all but a couple of us.  Instead, on
EINVAL only, I plan to output a much more user friendly string and I plan to
ratelimit the printk since many of these could be generated very rapidly.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>