275: add a write and reserve test

275: add a write and reserve test

This test is for write-posix test. If writing a file when the disk is almost
full, the posix wants the call to write as much as possible but not none.

quote the POSIX:
If a write() requests that more bytes be written than there is room for
(for example, [XSI] [Option Start] the process' file size limit or
[Option End] the physical end of a medium), only as many bytes as there
is room for shall be written. For example, suppose there is space for 20
bytes more in a file before reaching a limit. A write of 512 bytes will
return 20. The next write of a non-zero number of bytes would give a
failure return (except as noted below).

Signed-off-by: Wu Bo <Wu.Bo@cn.fujitsu.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>