f2fs: name gc task as per the block device

Currently GC task is started for each f2fs formatted/mounted device.
But, when we check the task list, using 'ps', there is no distinguishing
factor between the tasks. So, name the task as per the block device just
like the flusher threads.
Also, remove the macro GC_THREAD_NAME and instead use the name: f2fs_gc
to avoid name length truncation, as the command length is 16
-> TASK_COMM_LEN 16 and example name like:
f2fs_gc_task:8:16 -> this exceeds name length

Before Patch for 2 F2FS formatted partitions:
root  28061  0.0  0.0  0 0 ? S 10:31   0:00 [f2fs_gc_task]
root  28087  0.0  0.0  0 0 ? S 10:32   0:00 [f2fs_gc_task]

After Patch:
root  16756  0.0  0.0  0  0 ?  S  14:57   0:00 [f2fs_gc-8:18]
root  16765  0.0  0.0  0  0 ?  S  14:57   0:00 [f2fs_gc-8:19]

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>