xfs: Fix uninitialized variable in xfs_reflink_reserve_cow_range()

xfs: Fix uninitialized variable in xfs_reflink_reserve_cow_range()

with gcc 4.1.2:

    fs/xfs/xfs_reflink.c: In function xfs_reflink_reserve_cow_range:
    fs/xfs/xfs_reflink.c:327: warning: error may be used uninitialized in this function

Indeed, if "count" is zero, the function will return an uninitialized
error value.

While "count" is unlikely to be zero, this function is called through
the public iomap API. Hence fix this by preinitializing error to zero.

Fixes: 816a5196548b2b27 ("xfs: create delalloc extents in CoW fork")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>