LU-12678 o2iblnd: Use ib_mtu_int_to_enum()

LU-12678 o2iblnd: Use ib_mtu_int_to_enum()

Rather than bespoke code for converting an MTU into the enum,
use ib_mtu_int_to_enum().
This has slightly different behaviour for invalid values,
but those are caught when the parameter is set.

Test-Parameters: trivial testlist=sanity-lnet
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I9ba4230fd1aad2c6c59233bac9558870cb1ffeda
Reviewed-on: https://review.whamcloud.com/39123
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>