xfs_io: remove useless do_preadv and do_pwritev arguments

xfs_io: remove useless do_preadv and do_pwritev arguments

do_preadv and do_pwritev all have a 'buffer_size' argument, but they
never used it. Instead of it, they use global 'buffersize' variable,
which is initialized in alloc_buffer(). As the 'buffer_size' is
useless, so remove it for clear code.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>