libmount: improve mnt_split_optstr() performance

libmount: improve mnt_split_optstr() performance

This function is used by fstab (etc.) parser to split VFS, FS and
userspace options to separate lists. Unfortunately, the current
implementation reallocates the final string always when append a new
option to the string.

The new implementation pre-allocate memory for the final string
according to source string length (1/2 of the original string). This
dramatically reduces realloc() calls.

For example oss-fuzz (./test_mount_fuzz) uses 800K input string, old
version needs 28s to parse the string, new version 500ms.

Addresses: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=23861
Signed-off-by: Karel Zak <kzak@redhat.com>