performance/read-ahead: don't set ra_file in fd->ctx unless all memebers of ra_file is initialized

- If ptr to ra_file is set in fd->ctx even before initializing all its
    members, A race condition may occur b/w a thread executing ra_fstat,
    ra_readv etc (where all files open on the same inode are flushed) and
    the thread doing initialization of ra_file (in ra_open_cbk or
    ra_create_cbk). Because of this race-condition, flush_region might be
    called on an uninitialized ra_file, thereby causing crash.

Signed-off-by: Raghavendra G <raghavendra@gluster.com>
Signed-off-by: Anand V. Avati <avati@dev.gluster.com>

BUG: 868 (crash in ra_fstat)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=868

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