fs/btrfs: remove destination file before generation

fs/btrfs: remove destination file before generation

btrfs will happily use an existing destination file if it
already exists, increasing its size if needed. Hoever, it
will never decrease the size, even if the requested size
is smaller than the existing file.

So, remove any previously existing destination file before
generating the new filesystem.

Note: the original submission by Robert did that, but as
this case was not obvious, the removal was dropped by a
refactoring when the patch was initially applied.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Robert J. Heywood <robert.heywood@codethink.co.uk>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[Thomas: use $@ instead of $(@), use $(RM) instead of rm.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>