gfs2: Make sure not to return short direct writes

When direct writes fail with -ENOTBLK because we're writing into a
hole (gfs2_iomap_begin()) or because of a page invalidation failure
(iomap_dio_rw()), we're falling back to buffered writes.  In that case,
when we lose the inode glock in gfs2_file_buffered_write(), we want to
re-acquire it instead of returning a short write.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>