common/populate: fix bash syntax error in _fill_fs

common/populate: fix bash syntax error in _fill_fs

In bash, one does not set a variable by prepending the dollar sign to
the variable name.  Amazingly, this was copied verbatim from generic/256
in 2016 and hasn't been caught since its introduction in 2011. :(

Cc: allison.henderson@oracle.com
Fixes: 815015e9ee ("generic: make 17[1-4] work well when btrfs compression is enabled")
Fixes: b55fb0807c ("xfstests: Add ENOSPC Hole Punch Test")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>