zfs_inode_update should not call dmu_object_size_from_db under spinlock

We should never block when holding a spin lock, but zfs_inode_update can
block in the critical section of a spin lock in zfs_inode_update:

zfs_inode_update -> dmu_object_size_from_db -> zrl_add -> mutex_enter

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #3858