generic/015: Change the test filesystem size to 101mb

This test has been failing for btrfs for quite some time, at least
since 4.7. There are 2 implementation details of btrfs that it
exposes:

1. Currently btrfs filesystem under 100mb are created in Mixed block
group mode. Freespace accounting for it is not 100% accurate - I've
observed consistent 1mb discrepancy between a newly created
filesystem, then writing a file and deleting it and checking the
free space.

2. BTRFS won't flush it's delayed allocation on file deletion if
less than 32mb are deleted. On such files we need to perform sync
(missing in the test) or wait until time elapses for transaction
commit.

In order to avoid both of the aforementioned idiosyncrasies of the
fs make the test filesystem 101mb. With this we achieve 2 things:

1. Since the filesystem is larger we can create a file larger than
32mb, so it's going to be flushed upon deletion and numbers acquired
from df will be accurate
2. We don't create the filesystem in mixed mode and also since the
1mb is less than %1 of 101mb we will fall within the tolerance of 1%

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

tests/generic/015

@@ -53,7 +53,11 @@ _supported_os Linux
 _require_scratch
 _require_no_large_scratch_dev
 
-_scratch_mkfs_sized `expr 50 \* 1024 \* 1024` >/dev/null 2>&1 \
+# With filesystems less than 100mb btrfs is created in mixed mode
+# which can lead to slight accounting errors of 1mb. Having the
+# fs be at least 101 mb ensures those errors are within the error
+# tolerance of 1%
+_scratch_mkfs_sized `expr 101 \* 1024 \* 1024` >/dev/null 2>&1 \
     || _fail "mkfs failed"
 _scratch_mount || _fail "mount failed"
 out=$SCRATCH_MNT/fillup.$$