btrfs: add trace events for defrag

btrfs: add trace events for defrag

This patch will introduce the following trace events:

- trace_defrag_add_target()
- trace_defrag_one_locked_range()
- trace_defrag_file_start()
- trace_defrag_file_end()

Under most cases, all of them are needed to debug policy related defrag
bugs.

The example output would look like this: (with TASK, CPU, TIMESTAMP and
UUID skipped)

 defrag_file_start: <UUID>: root=5 ino=257 start=0 len=131072 extent_thresh=262144 newer_than=7 flags=0x0 compress=0 max_sectors_to_defrag=1024
 defrag_add_target: <UUID>: root=5 ino=257 target_start=0 target_len=4096 found em=0 len=4096 generation=7
 defrag_add_target: <UUID>: root=5 ino=257 target_start=4096 target_len=4096 found em=4096 len=4096 generation=7
...
 defrag_add_target: <UUID>: root=5 ino=257 target_start=57344 target_len=4096 found em=57344 len=4096 generation=7
 defrag_add_target: <UUID>: root=5 ino=257 target_start=61440 target_len=4096 found em=61440 len=4096 generation=7
 defrag_add_target: <UUID>: root=5 ino=257 target_start=0 target_len=4096 found em=0 len=4096 generation=7
 defrag_add_target: <UUID>: root=5 ino=257 target_start=4096 target_len=4096 found em=4096 len=4096 generation=7
...
 defrag_add_target: <UUID>: root=5 ino=257 target_start=57344 target_len=4096 found em=57344 len=4096 generation=7
 defrag_add_target: <UUID>: root=5 ino=257 target_start=61440 target_len=4096 found em=61440 len=4096 generation=7
 defrag_one_locked_range: <UUID>: root=5 ino=257 start=0 len=65536
 defrag_file_end: <UUID>: root=5 ino=257 sectors_defragged=16 last_scanned=131072 ret=0

Although the defrag_add_target() part is lengthy, it shows some details
of the extent map we get.
With the extra info from defrag_file_start(), we can check if the target
em is correct for our defrag policy.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>