nilfs2: replace nilfs_warning() with nilfs_msg()

Use nilfs_msg() to output warning messages and get rid of
nilfs_warning() function.  This also removes function names from the
messages unless we embed them explicitly in format strings.  Instead,
some messages are revised to clarify the context.

[arnd@arndb.de: avoid warning about unused variables]
  Link: http://lkml.kernel.org/r/20160615201945.3348205-1-arnd@arndb.de
Link: http://lkml.kernel.org/r/1464875891-5443-6-git-send-email-konishi.ryusuke@lab.ntt.co.jp


Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>