xfs: refactor realtime meta inode locking

Replace all the open-coded locking of realtime metadata inodes with a
single rtlock function that can lock all the pieces that the caller
wants in a single call.  This will be important for maintaining correct
locking order later when we start adding more realtime metadata inodes.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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