LU-12910 osc: allow increasing osc.*.short_io_bytes

LU-12910 osc: allow increasing osc.*.short_io_bytes

The osc.*.short_io_bytes parameter was mixing up the default and
maximum parameter values, and did not allow increasing the parameter
beyond the default.

Allow it to be increased to the maximum value, which depends on the
client PAGE_SIZE, and the amount of free space in the maximally-sized
OST RPC.  Since the maximum size is system dependent, allow some
grace when setting the parameter, so that a single tunable parameter
can work on a variety of different systems.

However, if it is larger than the maximum RDMA size (which is already
too large) return an error, as it means something is wrong.

Add a test case to exercise the osc.*.short_io_bytes parameter.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I2ce73af5963a0f9e0f1079dd2f91a4495a3ebbe5
Reviewed-on: https://review.whamcloud.com/36587
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>