Improve error handling for posix_fallocate(2) and posix_fadvise(2).

• Set td_errno so that ktrace and dtrace can obtain the syscall error number in the usual way.
• Pass negative error numbers directly to the syscall layer, as they're not intended to be returned to userland.

Reviewed by: kib
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D5425