linux: fix support for preadv/pwritev-less kernels

The fix from commit 269ff0be is wrong in that it sniffs system headers
to determine if preadv() and pwrite() are supported.

It breaks the fs_write_multiple_bufs test on one of my ARM systems;
its glibc has preadv() and pwritev() wrappers but its kernel has no
support for the corresponding system calls.

Simply try the system call and fall back to a normal read or write
loop if not supported.  Move the no_preadv and no_pwritev variables
around to squelch two -Wpedantic warnings about mixing declarations
with code.

R=@saghul