ceph: messenger cleanups and fixes

This series culminates in fixing a bug in which ceph connection header might get sent over the wire before its associated authorizer structure has been fully prepared. More info is found here:

    http://tracker.newdream.net/issues/2424

Sage preemptively reviewed them before I got them posted to the list, and the patches that follow indicate that. These patches are being tested now, and unless I get actionable feedback (and if they successfully pass testing) I'll be committing them as-is.

Along the way a few other small bugs are fixed (or avoided) and some data structures and interfaces are modified a bit to simplify things. Here is a bit of summary information about them.

These first four rearrange some places where a connection's out_kvec fields get reset, and where/when a banner gets put out prior to a connection header. A couple of functions can then eliminate a parameter as a result.
libceph: don't reset kvec in prepare_write_banner()
ceph: messenger: reset connection kvec caller
ceph: messenger: send banner in process_connect()
ceph: drop msgr argument from prepare_write_connect()

This defers setting WRITE_PENDING when a connection header has been queued to write but not its associated authorizer buffer.
ceph: don't set WRITE_PENDING too early

These add error checking in two spots, and rearranges a function so a simple case can be handled without dropping a connection's mutex.
ceph: messenger: check prepare_write_connect() result
ceph: messenger: rework prepare_connect_authorizer()
ceph: messenger: check return from get_authorizer

This defines a type to group some authorizer-related fields, then uses it to simplify some function interfaces. It also adds some additional checking before using method function pointers.
ceph: define ceph_auth_handshake type
ceph: messenger: reduce args to create_authorizer
ceph: ensure auth ops are defined before use
ceph: have get_authorizer methods return pointers
ceph: use info returned by get_authorizer
ceph: return pointer from prepare_connect_authorizer()

These final two implement the final fix for bug 2424 mentioned above. It doesn't place the connection header into out_kvec until it is fully initialized, and then ensures the associated authorizer buffer is also added before marking the WRITE_PENDING flag on the connection (and also before dropping the connection mutex).
ceph: rename prepare_connect_authorizer()
ceph: add auth buf in prepare_write_connect()

-Alex