LU-4208 osd-zfs: hold pool config lock to register property

LU-4208 osd-zfs: hold pool config lock to register property

- Hold the DSL pool configuration lock when calling
  dsl_prop_register().  Failure to do so will panic the node if
  assertions are enabled in ZFS.  This change requires a build of ZFS
  on Linux that exports symbols dsl_pool_config_enter and
  dsl_pool_config_exit, which was done in commit 40a806d [1], and will
  appear in ZFS release 0.6.3.

- Fix up variable declaration alignment in osd_mount().

- Add check for exported symbols in autoconf

[1] https://github.com/zfsonlinux/zfs/commit/40a806d

Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Ib2df05301ca73847d49ca2b183d72b9e2d957c10
Reviewed-on: http://review.whamcloud.com/8172
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Li Wei <wei.g.li@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>