fstests: do not call _scratch_mkfs_sized in a pipe

fstests: do not call _scratch_mkfs_sized in a pipe

_scratch_mkfs_sized will create a filesystem of the given size, and
call _notrun and exit if current $FSTYP doesn't support sized mkfs.
But when it's called in a pipe, the exit in _notrun only exits from
the subshell created by the pipe not the test itself, and test
continues to run unnecessarily, though the test is still reported as
[notrun] due to existence of $seqres.notrun file.

Fix it by not calling _scratch_mkfs_sized in a pipe, but dumping the
output to a tmp file, which will be fed to _filter_mkfs later.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>