fat: add fat_fallocate operation

fat: add fat_fallocate operation

Implement preallocation via the fallocate syscall on VFAT partitions.
This patch is based on an earlier patch of the same name which had some
issues detailed below and did not get accepted.  Refer
https://lkml.org/lkml/2007/12/22/130.

a) The preallocated space was not persistent when the
FALLOC_FL_KEEP_SIZE flag was set. It will deallocate cluster at evict time.

b) There was no need to zero out the clusters when the flag was set
Instead of doing an expanding truncate, just allocate clusters and add
them to the fat chain. This reduces preallocation time.

Compatibility with windows:
There are no issues when FALLOC_FL_KEEP_SIZE is not set
because it just does an expanding truncate. Thus reading from the
preallocated area on windows returns null until data is written to it.

When a file with preallocated area using the FALLOC_FL_KEEP_SIZE was
written to on windows, the windows driver freed-up the preallocated
clusters and allocated new clusters for the new data. The freed up
clusters gets reflected in the free space available for the partition
which can be seen from the Volume properties.

The windows chkdsk tool also does not report any errors on a
disk containing files with preallocated space.

And there is also no issue using linux fat fsck.
because discard preallocated clusters at repair time.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com>
Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>