alpha: propagate the calling convention changes down to csum_partial_copy.c helpers

get rid of set_fs() in csum_partial_copy_nocheck(), while we are at it -
just take the part of csum_and_copy_from_user() sans the access_ok() check
into a helper function and have csum_partial_copy_nocheck() call that.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>