selinux: remove pointless cast in selinux_inode_setsecurity()

security_context_to_sid() expects a const char* argument, so there's
no point in casting away the const qualifier of value.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <pmoore@redhat.com>