ext4: avoid calling ext4fs_mount() twice, which leaks

ext4_write_file() is only called from the "fs" layer, which calls both
ext4fs_mount() and ext4fs_close() before/after calling ext4_write_file().
Fix ext4_write_file() not to call ext4fs_mount() again, since the mount
operation malloc()s some RAM which is leaked when a second mount call
over-writes the pointer to that data, if no intervening close call is
made.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>