LU-11758 osp: remove assertion from statfs

LU-11758 osp: remove assertion from statfs

Sequence can't be changed or overflowed
in case of IDIF. Thus don't tigger kernel
panic for below case:
last_created [0x100000001:0x15:0x0], next_fid [0x100000000:0xfffffff6:0x0]
The same assertion that excepts IDIFs exists
in osp_fid_diff.
Also the patch is adding several optimizations
in osp_precreate_send.

Lustre-commit: bcfd0e040d1536410ba6c301f64d4f8ea6a8797a
Lustre-change: https://review.whamcloud.com/33832

Change-Id: I3966dfc621999d065c9b485d387938085fccb140
Cray-bug-id: LUS-2386
Signed-off-by: Sergey Cheremencev <c17829@cray.com>
Reviewed-by: Alexander Boyko <c17825@cray.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Reviewed-on: https://review.whamcloud.com/37993
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sergey Cheremencev <sergey.cheremencev@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>