mds: Return created inode in mds reply to create

mds: Return created inode in mds reply to create

If multiple clients race to create a file, multiple clients will send a
create request and get back a valid dentry+inode, but only one client
will actually win the race to create the file.  All other clients should
treat the reply as an open of an existing file and check permissions.
This patch adds the created inode number to the mds create reply if that
request actually created the inode/file (and the feature is supported),
so the client can properly check permissions if the inode number isn't
returned.  Fixes #3625.

Signed-off-by: Sam Lang <sam.lang@inktank.com>