btrfs: properly report 0 avail for very full file systems

stable inclusion
from stable-v5.10.198
commit cbbfdb4bab700a72f681de2e49a8c22780b2bdf9
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I987V5

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cbbfdb4bab700a72f681de2e49a8c22780b2bdf9



--------------------------------

commit 58bfe2cc upstream.

A user reported some issues with smaller file systems that get very
full.  While investigating this issue I noticed that df wasn't showing
100% full, despite having 0 chunk space and having < 1MiB of available
metadata space.

This turns out to be an overflow issue, we're doing:

  total_available_metadata_space - SZ_4M < global_block_rsv_size

to determine if there's not enough space to make metadata allocations,
which overflows if total_available_metadata_space is < 4M.  Fix this by
checking to see if our available space is greater than the 4M threshold.
This makes df properly report 100% usage on the file system.

CC: stable@vger.kernel.org # 4.14+
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>