examples/userfs: use designated initializers for struct dirent

[xiaoxiang781216]

Summary

The userfs example initialized each g_rootdir[] entry's
struct dirent member positionally as { DTYPE_FILE, "FileN" }, which
assumes d_type is the first field of struct dirent.

apache/nuttx PR #19179 ("fs: widen ino_t to uint32_t and add d_ino to
struct dirent") adds the POSIX d_ino member as the first field of
struct dirent. With that change the positional initializer assigns the
file-name string to d_type (a uint8_t), producing
-Wint-conversion warnings and an
error: initializer element is not computable at load time, which broke
the CI build.

This change converts the three initializers to designated initializers
(.d_type / .d_name) so the table is robust against the field order of
struct dirent.

Impact

• examples/userfs only. No functional change; the same d_type/d_name
  values are assigned. d_ino is left zero-initialized, which is fine for
  this in-memory test filesystem.

Testing

• checkpatch.sh -f examples/userfs/userfs_main.c → all checks pass.
• A standalone host program reproducing the table against a d_ino-first
  struct dirent compiles cleanly with -Wall -Wextra -Werror and assigns
  d_type=DTYPE_FILE, d_name="File1" as expected.

Related

• Companion to fs: widen ino_t to uint32_t and add d_ino to struct dirent nuttx#19179. That PR's CI build fails on the current
  positional initializer; this PR fixes it. They should be merged together.

[Copilot]

Pull request overview

This PR updates the examples/userfs in-memory directory table to use designated initializers for struct dirent so it remains correct even if the field order changes (e.g., with the addition of d_ino).

Changes:

• Converted g_rootdir[] struct dirent initializers from positional to designated (.d_type, .d_name) for File1–File3.

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