e2fsck: avoid mallinfo() if over 2GB allocated

e2fsck: avoid mallinfo() if over 2GB allocated

Don't use mallinfo() for determining the amount of memory used if it
is over 2GB.  Otherwise, the signed ints used by this interface can
can overflow and return garbage values.  This makes the actual amount
of memory used by e2fsck misleading and hard to determine.

Instead, use brk() to get the total amount of memory allocated, and print
this if the more detailed mallinfo() information is not suitable for use.

There does not appear to be a mallinfo64() variant of this function.
There does appear to be an abomination named malloc_info() that writes
XML-formatted malloc stats to a FILE stream that would need to be read
and parsed in order to get these stats, but that doesn't seem worthwhile.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Shilong Wang <wshilong@ddn.com>
Lustre-bug-id: https://jira.whamcloud.com/browse/LU-13197
Signed-off-by: Theodore Ts'o <tytso@mit.edu>