afr: use data trylock mode in read/write self-heal trigger paths

Self-heal data lock contention between clients and glustershd
instances can lead to long wait and user response times if the
client ends up pending its lock on glustershd self-heal of a large
file. We have reports of guest vm instances going completely
unresponsive during self-heal of virtual disk images.

Optimize the read/write self-heal trigger codepath
(i.e., afr_open_fd_fix()) to trylock for self-heal and skip the
self-heal otherwise to minimize the likelihood of a running/active
guest of competing with glustershd on arrival of a brick. Note that
lock contention is still possible from the client (e.g., via
lookup).

BUG: 874045
Change-Id: I406443c061ff6acd2a851179626b78352caa5c03
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-on: http://review.gluster.org/4258
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>

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