LU-7898 osd: remove unnecessary declarations

Refactor the code a bit to remove unnecessary declarations
(which are very expensive in ZFS). The patch also introduces
initial preparations to support large dnodes - it tracks
all declared EAs at object creation and tracked number can
be used to request dnode of appropriate size.

With this patch + LU-7918 disk/memory space reserved for a
single-stripe creation goes down from ~33MB to 4.6MB.

Performance improvements from this patch are also significant.
Running mdtest create performance on a test node (ramdisk):

    Threads    0.6.5   0.6.5+patch
        1       9933       14279
        2      12870       20469
        4      16405       26407
        8      19320       28254
       16      15648       26620
       32      14107       26483

Change-Id: I2c25542e51a320b1b48b4782b5f0b43799de5fe9
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: http://review.whamcloud.com/19101
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/22296