src/seek_sanity_test: ensure file size is big enough

src/seek_sanity_test: ensure file size is big enough

Tests test07, test08, and test09 preallocate a file and assume the
file size used is bigger than 10xbufsz (100xbufsz for test09).  This
patch adjusts the file size so this assumption is always true.

As an example, here's test07 output for cephfs, where the allocation
size is set to 4194304, and the output is (4194304 * 10 + 4194304)

  07. Test file with unwritten extents, only have dirty pages
  07.01 SEEK_HOLE expected 0 or 4194304, got 46137344.              FAIL
  07.02 SEEK_HOLE expected 1 or 4194304, got 46137344.              FAIL

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>