Set mtime on symbolic links

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