lint: silence clippy::unnecessary_cast on glibc + fix doc list indent

CI's lint workflow runs clippy on linux target where:
- platform/fs_type/linux.rs and io/writeback/linux.rs: the i64 cast
  on buf.f_type / NFS_SUPER_MAGIC is unnecessary on glibc x86_64 (both
  already i64) but required on musl (c_ulong); silence the lint via
  inline allow with explanatory comment.
- mux/m2ts_mux/packet.rs: doc comment continuation across lines was
  parsed as an unindented list item. Reworded to a single flowing
  sentence.

src/mux/m2ts_mux/packet.rs

@@ -57,11 +57,11 @@ impl Packet {
 
     /// Append the adaptation field after the header.
     ///
-    /// `body` is the adaptation field body (flags byte + optional PCR
-    /// + …). `stuffing` is the number of `0xFF` stuffing bytes to
+    /// `body` is the adaptation field body (flags byte plus optional PCR
+    /// and so on). `stuffing` is the number of `0xFF` stuffing bytes to
     /// append after the body. The first byte of the field
-    /// (`adaptation_field_length`) is computed here from `body.len() +
-    /// stuffing`.
+    /// (`adaptation_field_length`) is computed here from
+    /// `body.len() + stuffing`.
     pub(super) fn append_adaptation(&mut self, body: &[u8], stuffing: usize) {
         let af_len = body.len() + stuffing;
         debug_assert!(af_len <= 183, "adaptation field overflow");