fat write: Fixed a problem with the case of file names when writing files

commit 21a24c3b ("fs/fat: fix case for FAT shortnames") made it
possible that get_name() returns file names with some upper cases.
find_directory_entry() must be updated to take this account, and use
case-insensitive functions to compare file names.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>