nvram: fix memory leak

nvram: fix memory leak

Fix memory leak on nvram_open() and nvram_open_rdonly().

For nvram_open(), the 'fd' should be closed on error, and
mmap_area should be unmap when nvram magic can not be found.

For nvram_open_rdonly(), the 'file' variable should free before
return. Once nvram_find_mtd() return successfully, it will allocate
memory to save mtd device string.

Signed-off-by: BangLang Huang <banglang.huang@foxmail.com>