Do not update the fsinfo block on each update of any fat block, this
is excessive. Postpone the flush of the fsinfo to VFS_SYNC(),
remembering the need for update with the flag MSDOSFS_FSIMOD, stored
in pm_flags.

FAT32 specification describes both FSI_Free_Count and FSI_Nxt_Free as
the advisory hints, not requiring them to be correct.

Based on the patch from bde, modified by me.

Reviewed by: bde
MFC after: 2 weeks