test_librbd_fsx: use posix_memalign() to allocate aligned buffers

test_librbd_fsx: use posix_memalign() to allocate aligned buffers

Use posix_memalign() to allocate good_buf and temp_buf, which must be
writebdy and readbdy aligned respectively.  Using round_ptr_up() the
way it is used makes fsx crash on free()s at the end of main(), because
the pointer returned by malloc() is overwritten by the aligned pointer.

Drop round_ptr_up() as it is no longer used.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>