LU-14732 utils: ensure hsm_user_request extent fields are zero

LU-14732 utils: ensure hsm_user_request extent fields are zero

Another patch changed the linking flags for liblustreapi which
caused sanit-hsm 29c to fail. Debugging revealed the reason was
the extent fields in hsm_user_request sent to the kernel was
not zeroed out. For some reason the compiler flags hide this
bug. I found this is easily reproduced by removing
lhsmtool_posix_DEPENDENCIES which is not needed anyways since
the build system can calculate those dependencies for us.
Update the function llapi_hsm_user_request_alloc to zero out
the struct hsm_user_request allocated.

Change-Id: I02c1d6b5cec1ed3e89086a6a00e30ca81768409c
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/43893
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Ben Evans <beevans@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>