splitmuxsrc: Protect initial pad configuration with the object lock

gst_splitmux_src_activate_part() configures the pad information
before starting the pad task, but occasionally the changes it makes
to the pad are not seen in the pad task because they're not
protected by the right locking. Use the pad's object lock to
protect those variables.
18 jobs for splitmuxsrc-minor in 72 minutes and 51 seconds (queued for 35 seconds)
Status Job ID Name Coverage
  Preparation
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gst indent

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  Build
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build fedora x86_64

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  Full Builds
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gst-macos-10.14
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  Full Tests
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  Apps
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  Deploy
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documentation

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