Btrfs: don't abort the current transaction if there is no enough space for inode...

Btrfs: don't abort the current transaction if there is no enough space for inode cache

The filesystem with inode cache was forced to be read-only when we umounted it.

Steps to reproduce:
 # mkfs.btrfs -f ${DEV}
 # mount -o inode_cache ${DEV} ${MNT}
 # dd if=/dev/zero of=${MNT}/file1 bs=1M count=8192
 # btrfs fi syn ${MNT}
 # dd if=${MNT}/file1 of=/dev/null bs=1M
 # rm -f ${MNT}/file1
 # btrfs fi syn ${MNT}
 # umount ${MNT}

It is because there was no enough space to do inode cache truncation, and then
we aborted the current transaction.

But no space error is not a serious problem when we write out the inode cache,
and it is safe that we just skip this step if we meet this problem. So we need
not abort the current transaction.

Reported-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Tested-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>