_IO_file_underflow_mmap read call for atime

[Roland McGrath]

The POSIX rules for mmap on files require that mmap'ing and reading a page
set the atime.  I don't see how the rules for stdio affecting atime (via
the "underlying functions" clauses) could be violated by the libio mmap'd
buffer implementation if mmap behaves as specified.  So the read call in
_IO_file_underflow_mmap is unnecessary.  Am I missing something?