sym(4): fix nits reported by Coverity

sym(4): fix nits reported by Coverity

- In ___dma_getp(), remove dead code. [1]
- In sym_sir_bad_scsi_status(), add missing FALLTHROUGH. [2]

While at it:
- For getbaddrcb(), remove __unused from the nseg argument as it's in
  fact used when compiling with INVARIANTS.
- In sym_int_sir(), ensure in all branches that cp is not NULL before
  using it.

Reported by:	Coverity
CID:		1008861 [1], 114996 [2]