Expose cvmx_mgmt_port_num_ports() as a wrapper to the static
__cvmx_mgmt_port_num_ports() mostly to avoid exporting a function
starting with __.
Use cvmx_mgmt_port_num_ports() instead of the inlined copy of a
previous version of this function.
The fsync(2) call should sync the vnode in such way that even after
system crash which happen after successfull fsync() return, the data
is accessible.  For msdosfs, this means that FAT entries for the file
must be written.

Since we do not track the FAT blocks containing entries for the
current file, just do a sloppy sync of the devvp vnode for the mount,
which buffers, among other things, contain FAT blocks.

Simultaneously, for deupdat():
- optimize by clearing the modified flags before short-circuiting a
  return, if the mount is read-only;
- only ignore the rest of the function for denode with DE_MODIFIED
  flag clear when the waitfor argument is false.  The directory buffer
  for the entry might be of delayed write;
- microoptimize by comparing the updated directory entry with the
  current block content;
- try to cluster the write, fall back to bawrite() if low on
  resources.

Based on the submission by:	bde
MFC after:	2 weeks