securityfs: use kstrdup_const() to manage symlink targets

Since 'target' argument of 'securityfs_create_symlink()' is (for
now at least) a compile-time constant, it may be reasonable to
use 'kstrdup_const()' / 'kree_const()' to manage 'i_link' member
of the corresponding inode in attempt to reuse .rodata instance
rather than making a copy.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Paul Moore <paul@paul-moore.com>