xfs: Kill the hardcode number 128 for transactions space reservation

Hello,

We have a hard code number of "128" for almost all sorts of transactions space reservations. This number is used to reserve the space for the struct xfs_buf_log_format that gets written into the log for every buffer as well as a little bit of extra space because there also needs a log opheader, i.e. struct xlog_op_header.

This idea is totally inspired by Dave's comments at:
http://oss.sgi.com/archives/xfs/2012-12/msg00009.html
I'll add Signed-off-by for Dave once the patches are qualified to be accept for his credit.

The patches replace the magic number with a new function xfs_buf_log_overhead(), which is introduced to examine the extra buf log space more flexible than hard coding it.

Maybe it's better to tweak it up in a macro to make the code style in xfs_trans.c looks a bit neat? #define XFS_BUF_LOG_OVERHEAD (xfs_buf_log_overhead())

To make sure there is no regression with those changes, I have run dozens of xfs test cases, they are works to me.

[PATCH 1/4] Add a new function xfs_buf_log_overhead() to replace the hard-code number of 128 [PATCH 2/4] Make xfs_fs_log_dummy() aware of this change
[PATCH 3/4] Make super block unit change related transactions aware of this change [PATCH 4/4] Make disk quota related transactions aware of this change

---
 fs/xfs/xfs_dquot.c       |    6 ++-
 fs/xfs/xfs_fsops.c       |    5 +-
 fs/xfs/xfs_mount.c       |   15 +++---
 fs/xfs/xfs_qm.c          |    6 +--
 fs/xfs/xfs_qm_syscalls.c |   20 ++++----
 fs/xfs/xfs_trans.c       |  113 ++++++++++++++++++++++++++++++----------------
 fs/xfs/xfs_trans.h       |    4 +-
 7 files changed, 106 insertions(+), 63 deletions(-)

Thanks,
-Jeff

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