selinux: hooks: use __getname() to allocate path buffer

selinux: hooks: use __getname() to allocate path buffer

selinux_genfs_get_sid() allocates memory for a path with __get_free_page()
although there is a dedicated helper for allocation of file paths:
__getname().

Replace __get_free_page() for allocation of a path buffer with __getname().

Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>