bpf-restrict-fs: use a 32-bit magic key on big-endian too

bpf-restrict-fs: use a 32-bit magic key on big-endian too

The inner map is created with a uint32_t key, but the update passed
&magic[i] where magic is a (possibly 64-bit) statfs_f_type_t. On
little-endian the low 32 bits happen to be read; on big-endian 64-bit
(s390x, ppc64 BE) the zero high word is read instead, so every
filesystem collides on key 0 (the allow/deny selector) and
RestrictFileSystems= is silently broken. Pass a truncated copy.

Follow-up for 184b4f78cfbded54a6e06bbe1152256c204a7a73