mds: get_linkage() should return projected if xlocked by self

mds: get_linkage() should return projected if xlocked by self

Also, move set_xlocks_done() to _below_ set_trace_dist, in case some
other request comes in and xlocks the dentry.

Allow read_projected in XLOCK state too.  Note that other requests
_still_ won't be able to re-xlock or rdlock until the prior request
set xlock done.