xfs: Fix mp->m_maxagi update during inode64 remount

With the changes made on xfs_set_inode64(), to make it behave as
xfs_set_inode32() (now leaving to the caller the responsibility to update
mp->m_maxagi), we use the return value of xfs_set_inode64() to update
mp->m_maxagi during remount.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>