Linux 4.7 compat: inode_lock() and friends

Linux 4.7 compat: inode_lock() and friends

Linux 4.7 changes i_mutex to i_rwsem, and we should used inode_lock and
inode_lock_shared to do exclusive and shared lock respectively.

We use spl_inode_lock{,_shared}() to hide the difference. Note that on older
kernel you'll always take an exclusive lock.

We also add all other inode_lock friends. And nested users now should
explicitly call spl_inode_lock_nested with correct subclass.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue zfsonlinux/zfs#4665
Closes #549