fs: exfat: Rework exfat_fs_readdir() to behave like exfat_fs_ls()

fs: exfat: Rework exfat_fs_readdir() to behave like exfat_fs_ls()

The exfat_fs_readdir() depends on state created in exfat_fs_opendir(),
but that state may be disrupted by fs_close() called by the FS layer
in fs_opendir(), because exfat porting layer unmounts the filesystem
in ->close() callback.

To avoid this disruption, avoid creating state in exfat_fs_opendir(),
cache only the directory name to list there, and rework exfat_fs_readdir()
to work in a similar way to exfat_fs_ls(). That is, make exfat_fs_readdir()
open the directory, look up specific entry, extract its properties to be
reported to FS layer, and close the directory. This is slow, but avoids
the disruption. The slowness does not affect regular 'ls' command, which
uses exfat_fs_ls() fast path.

Fixes: b86a651b646c ("fs: exfat: Add U-Boot porting layer")
Signed-off-by: Marek Vasut <marex@denx.de>