Fix lockdep warning in spl_inode_{lock,unlock}

Fix lockdep warning in spl_inode_{lock,unlock}

spl_inode_{lock,unlock} are triggering possible recursive locking
warnings from lockdep.  The warning is a false positive.

The lock is used to protect a parent directory during delete/add
operations, used in zfs when writing/removing the cache file.  The inode
lock is taken on both the parent inode and the file inode.

VFS provides an enum to subclass the lock.  This patch changes the
spin_lock call to _nested version and uses the provided enum.

Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #480