ext4: mark ctx_*_flags() with __maybe_unused

ext4: mark ctx_*_flags() with __maybe_unused

When ctx_set_flags() is unused, it prevents kernel builds
with clang, `make W=1` and CONFIG_WERROR=y:

.../ext4/super.c:2120:1: error: unused function 'ctx_set_flags' [-Werror,-Wunused-function]
 2120 | EXT4_SET_CTX(flags); /* set only */
      | ^~~~~~~~~~~~~~~~~~~

Fix this by marking ctx_*_flags() with __maybe_unused
(mark both for the sake of symmetry).

See also commit 6863f5643dd7 ("kbuild: allow Clang to find unused static
inline functions for W=1 build").

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20240905163229.140522-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>