bcachefs: Fix missing spaces in journal_entry_dev_usage_to_text

bcachefs: Fix missing spaces in journal_entry_dev_usage_to_text

Fixed missing spaces displayed in journal_entry_dev_usage_to_text
while adjusting the display format to improve readability.

before:
```
 # bcachefs list_journal -a -t alloc:1:0 /dev/sdb
 ...
     dev_usage: dev=0free: buckets=233180 sectors=0 fragmented=0sb: buckets=13 sectors=6152 fragmented=504journal: buckets=1847 sectors=945664 fragmented=0btree: buckets=20 sectors=10240 fragmented=0user: buckets=1419 sectors=726513 fragmented=15cached: buckets=0 sectors=0 fragmented=0parity: buckets=0 sectors=0 fragmented=0stripe: buckets=0 sectors=0 fragmented=0need_gc_gens: buckets=0 sectors=0 fragmented=0need_discard: buckets=1 sectors=0 fragmented=0
```

after:
```
 # bcachefs list_journal -a -t alloc:1:0 /dev/sdb
 ...
     dev_usage: dev=0
       free: buckets=233180 sectors=0 fragmented=0
       sb: buckets=13 sectors=6152 fragmented=504
       journal: buckets=1847 sectors=945664 fragmented=0
       btree: buckets=20 sectors=10240 fragmented=0
       user: buckets=1419 sectors=726513 fragmented=15
       cached: buckets=0 sectors=0 fragmented=0
       parity: buckets=0 sectors=0 fragmented=0
       stripe: buckets=0 sectors=0 fragmented=0
       need_gc_gens: buckets=0 sectors=0 fragmented=0
       need_discard: buckets=1 sectors=0 fragmented=0
```
Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>