gfapi: Suspend synctasks instead of blocking them

There are certain conditions which blocks the current
execution thread (like waiting on mutex lock or condition
variable or I/O response). In such cases, if it is a
synctask thread, we should suspend the task instead
of blocking it (like done in SYNCOP using synctask_yield)

This is to avoid deadlock like the one mentioned below -

1) synctaskA sets fs->migration_in_progress to 1 and
   does I/O (LOOKUP)
2) Other synctask threads wait for fs->migration_in_progress
  to be reset to 0 by synctaskA and hence blocked
3) but synctaskA cannot resume as all synctask threads are blocked
   on (2).

Note: this same approach is already used by few other components
like syncbarrier etc.

Change-Id: If90f870d663bb242c702a5b86ac52eeda67c6f0d
Fixes: #1146
Signed-off-by: Soumya Koduri <skoduri@redhat.com>

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