osd: don't require RWEXCL lock for stat+write ops.

osd: don't require RWEXCL lock for stat+write ops.
In librbd, a stat op is inserted before write op for cloned image. OSD used to use RWEXCL and such requests are processed one by one.
With this fix, OSD will use RWWRITE rather than RWEXCL for such [stat,write] request to allow multiple [stat,write] on the same object and improve performance.

Signed-off-by: Alice Zhao <brucen1030@163.com>
(cherry picked from commit 9be266b0a0304aaaaa0ca12f28fcd3e9cce1f9d7)