fs/fat: Check malloc return values and fix memory leaks

fs/fat: Check malloc return values and fix memory leaks

Check malloc() return values and properly unwind on errors so
memory allocated for fat_itr structures get freed properly.

Also fixes a leak of fsdata.fatbuf in fat_size().

Fixes: d32239991fb983 ("fs/fat: Reduce stack usage")
Reported-by: Coverity (CID: 167225, 167233, 167234)
Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Reviewed-by: Tom Rini <trini@konsulko.com>