f2fs/gc: Reduce GC thread urgent sleep time to 50ms

Android sets the value to 50ms via vold's IdleMaint service. Since
500ms is too long for GC to colllect all invalid segments in time
which results in performance degradation.

On un-encrypted device, vold fails to set this value to 50ms thus
degrades the performance over time.

Based on [1].

[1] https://github.com/topjohnwu/Magisk/pull/5462


Signed-off-by: Panchajanya1999 <rsk52959@gmail.com>
Change-Id: I80f2c29558393d726d5e696aaf285096c8108b23
Signed-off-by: improbir <ghosh_probir@yahoo.com>