selinux: fix sel_read_bool() allocation and error handling

Switch sel_read_bool() from using get_zeroed_page() and free_page()
to a stack-allocated buffer. This also fixes a memory leak in the
error path when security_get_bool_value() returns an error.

Reported-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>