LU-17307 mdt: get dirent count by request

LU-17307 mdt: get dirent count by request

Add MA_DIRENT_CNT/LA_DIRENT_CNT to notify osd to get dirent count.
Set it in mdt_getattr_name_lock() and when auto-split is enabled so it
won't cause overhead when auto-split is disabled, and change
oo_dirent_count type to atomic_t so the result does not become
inaccurate over time from repeated addition/removal (which may
be used to know whether directory is empty or compare directories in
the future).

In osd_dirent_count() set oo_dirent_count to 0 before iteration to
avoid multiple threads iterate at the same time, which means the
result may not be accurate in this case, but it will be eventually.

Fixes: 03a4431dac ("LU-11025 osd: osd_attr_get() returns dirent count")
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I2be6c0dcfda1c98995a269585c5d8d781a8a3b42