Fix profile_open_file() last_stat locking

Fix profile_open_file() last_stat locking

Commit 53d6d0d64e96b84b57b7446a787e683312e2a529 changed
profile_open_file() to force a stat() when reusing a profile object,
by setting the last_stat field to 0 on the data object.  The last_stat
field is controlled by the data object mutex (unlike the refcount
field, which is controlled by g_shared_trees_mutex).  Lock the data
object before setting last_stat.

[ghudson@mit.edu: switched to using profile_update_data_file_locked()
so we don't lock and unlock the data object twice; rewrote commit
message]

ticket: 9184 (new)