GFS2: combine duplicated block freeing routines

__gfs2_free_data and __gfs2_free_meta are almost identical, and
can be trivially combined.

[This is as per Eric's original patch minus gfs2_free_data() which had
 no callers left and plus the conversion of the bmap.c calls to these
 functions. All in all, a nice clean up]

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>