nilfs2: refactor nilfs_segctor_thread()

nilfs2: refactor nilfs_segctor_thread()

Simplify nilfs_segctor_thread(), the main loop function of the log writer
thread, to make the basic structure easier to understand.

In particular, the acquisition and release of the sc_state_lock spinlock
was scattered throughout the function, so extract the determination of
whether log writing is required into a helper function and make the
spinlock lock sections clearer.

Link: https://lkml.kernel.org/r/20240826174116.5008-9-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: Huang Xiaojia <huangxiaojia2@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>