fat: additions to support fat_fallocate

fat: additions to support fat_fallocate

Implement preallocation via the fallocate syscall on VFAT partitions.

With FALLOC_FL_KEEP_SIZE, there is no way to distinguish if the mismatch
between i_size and no.  of clusters allocated is a consequence of
fallocate or just plain corruption.  When a non fallocate aware (old)
linux fat driver tries to write to such a file, it throws an error.Also,
fsck detects this as inconsistency and truncates the prealloc'd blocks.

To avoid this, as suggested by OGAWA, remove changes that make fallocate
persistent across mounts and restrict lifetime of blocks from fallocate(2)
to file release.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Ravishankar N <ravi.n1@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>