Set mtime on symbolic links

Register the setattr/getattr callbacks for symlinks.  Without these
the generic inode_setattr() and generic_fillattr() functions will
be used.  In the setattr case this will only result in the inode being
updated in memory, the dirty_inode callback would also normally run
but none is registered for zfs.

The straight forward fix is to set the setattr/getattr callbacks
for symlinks so they are handled just like files and directories.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #412

module/zfs/zpl_inode.c

@@ -354,6 +354,8 @@ const struct inode_operations zpl_symlink_inode_operations = {
 	.readlink	= generic_readlink,
 	.follow_link	= zpl_follow_link,
 	.put_link	= zpl_put_link,
+	.setattr	= zpl_setattr,
+	.getattr	= zpl_getattr,
 };
 
 const struct inode_operations zpl_special_inode_operations = {