kldxref: Simplify elf_read_raw_data

Use pread as a valid offset is always passed now. Originally the DSO
code read the .hash section in two separate requests and relied on the
implicit offset for the second read, but now the hash table is fetched
in a single call.

Reviewed by: imp
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D43125

(cherry picked from commit ed96fd7fc652d77ae5e34727e54610e87854defc)