LU-13197 e2fsck: consistently use ext2fs_get_mem()

LU-13197 e2fsck: consistently use ext2fs_get_mem()

Consistently use ext2fs_get_mem() and ext2fs_free_mem() instead of
calling malloc() and free() directly in e2fsck.  In several places
it is possible to use ext2fs_get_memzero() instead of explicitly
calling memset() on the memory afterward.

This is just a code cleanup, and does not fix any specific bugs.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I1c0943da43299c187936bb28db5423ce3e3ebbe5
Reviewed-on: https://review.whamcloud.com/37441
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>