LU-4611 osd: improve credits calculation

LU-4611 osd: improve credits calculation

- llog catalog do not declare records twice (for old and new objects)
  this might be an issue on ZFS with full debug enabled..
- llog to specify append by pos=-1, so OSD can take this into account
- object create/destroy should not include OI, this is calculated yet
- EA declaration to improve few specific cases
- osd_declare_write() to recognize overwrite optimistically,
  using inode size and i_blocks
- osd_declare_write() to optimize very specific cases, like legacy
  blockmaps with small offsets and allocated indirects
- index delete modify just a single block

preliminary testing on a local setup with 7 OSTs:
1360 credits before and 436 credits after.

llog declarations and index inserts (part of llog object creation)
still consume about 70% (7 OSTs):
   create: 7/28, destroy: 1/4
   attr_set: 2/2, xattr_set: 8/21
   write: 37/179, punch: 14/56, quota 2/2
   insert: 8/135, delete: 2/5
   ref_add: 1/1, ref_del: 3/3

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: Icb5c79df1f8ba248509b9d2561ac8843bb01f6af
Reviewed-on: http://review.whamcloud.com/9258
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>